When is the right time to start farming the epic jungle mobs?

The straight answer is, as soon as you can and as often as you can.

The Red and Blue Buff, a.k.a. Lizard Elder (being Red) and Anciet Golem (being Blue), on your teams side of the map should be farmed immediately upon it spawning. Lets briefly touch what each buff does for your team:

Red

Most Jungler Champs will get the Red buff second to the Blue buff, as they require mana to quickly clear Creep Camps. Jungler Champs such as: Lee Sin, and Shyvana tend to go red first as it is most beneficial to them clearing Creep Camps. The Red buff also provides a huge advantage to the ADC/Top Laners, although most Junglers dont give up the Red buff till the laning phase is over. So making sure it is constantly in use is completely beneficial to your team and can generally be solo'd by Meta Jungle Champs at level 1.

Blue

As stated previously the Blue buff provides champs a steady amount of mana, so they can burn through their rotations without having to worry to much about being mana starved. Most Junglers will usually allow their mid to have this buff upon its respawn, giving them huge sustaining power for a short period of time and can definitely be the difference between winning mid lane or losing it. This buff should also be grabbed everytime it is up and is easily solo'd by Meta Jungle Champs at level 1.

My Two Cents

As someone who jungles occasionally with Udyr, I usually go for a fast Invade on their red (preferably unseen). Grabbing my Blue fast then moving on to handing over red to w.e lane is closest. This provides a nice little boost to either top or bot lane (these lanes failing tends to have the biggest impact on the outcome of the match and a mid has very little use for Red.) Doing this successfully will win you the game. Now not only is their jungle gonna be behind clearing Creep Camps, but one of your lanes should be winning. This will allow you to sit between the other two lanes and gank over and over while either stealing the enemy camps (since you will be stronger then their jungle) or safely farming your own.

Dragon and Baron are very tricky and when you should grab them generally depends on how well your team is doing.

Dragon

Killing the Dragon for your team can bring you back from a deficate or put you in a sizable lead. The importance of the Dragon is serious business, as their is no way for your enemies team to gain that gold once you've scored the kill. What I mean is you can take a tower, but all your enemy has to do is take your tower and they are even with you. Same goes for kills, anyone can get a kill because your enemy decides to constantly get out of position. Now getting a Dragon kill, thats the way to help your team. In short 190 gold for your team is a huge advantage, maybe not game winning immediately, but keep getting dragon 2-3 times in a row and the game is yours.

When to get Dragon, is a little tricky. Never get Dragon without knowing the location if the entire enemy team. For Example if you see them make a 4 man gank up top this is a great time to grab the dragon, or you manage to counter a gank without losing anyone killing off atleast the jungler and adc this is another instance to grab the Dragon. If you would like to sneak in a Dragon make sure you have a vision ward to remove the wards or someone with Oracles, a few minutes before you attempt it. The action of taking the Dragon should always be done with the Jungler (atleast early game) and should be quick.

Baron

Baron, for most part is just a game ender. Obviously if you are losing and manage to get Baron it can very easily change the tide of battle. But in reality most teams can't manage a Baron kill unless they are dominating the enemy, and do it after crushing the enemy in a team fight. Occasionally you can manage a sneak Baron, or steal it or something but be reasonable, it shouldn't be your game plan. So yah, take Baron when your team has pushed the enemy inside their base or come out of a team fight in good shape. And never attempt to sneak a Baron kill without checking for enemy wards, its a recipe for disaster.

And yes, they are all very important to kill, games are won and lost over the buffs and gold they provide.

If by "epic jungle mobs", you mean the Ancient Golem and Lizard Elder (the ones with the colored stones floating around them), you should always get them. They give buffs to the champion that kills them. Ancient Golem generally goes to your AP carry for increased mana regeneration and cooldown reduction. Lizard Elder, on the hand, is generally more efficient on either your bruiser or AD carry since it grants on-hit slows and burn effects.

If by "epic jungle mobs", you mean Baron Nashor and the Dragon (the mobs along the river), then it depends. Dragon should taken as soon as it is safe to do so, as it grants bonus gold to everyone on the team that kills it. Baron Nashor, on the other hand, must be killed carefully. It is a very strong monster will often take upwards of three champions to kill (sometimes requiring a team effort). Due to this, Baron is often contested and an ace at Baron usually results in a loss for the aced team. Killing Baron gives a team buff that increases damage dealt.

In summation:

Ancient Golem (Big rock dude with floating blue stones): Gives buff that dramatically increases mana regeneration and cooldown reduction.

Lizard Elder (Giant lizard thing with floating red stones): Gives buff that makes basic attacks slow and deal burn damage (this damage is periodic and true).

Dragon (... kinda self-explanatory): Gives 190 gold to the team that kills it.

Baron Nashor (giant purple worm of DOOM): Gives 300 gold to the team that kills it (as well as 25 gold to the champion that gets the last hit) along with a buff that increases AD and AP as well as health and mana regeneration.

Credit to Shykin for correcting me on dragon gold and providing additional information on Baron.

Here's a couple examples of good times to do either.

Dragon (Laning Phase): [Side note, I've successfully taken dragon as Warwick at only lvl 4 with a standard jungle build. Other sustain/damage junglers may also do that but it is risky. Anyways], If the jungler just ganked bot lane forcing one or both of the enemies to die/retreat. Go for dragon as it will most likely be uncontested (3 of you there with mid nearby as well vs. their jungler and mid if they go to dragon).

Dragon (Post-laning Phase): If most of the enemy team is away from the bottom/mid lane you could go for it. The best time however would be after you killed the enemy teams main damage (ADC/top/jungler) in a teamfight or gank. This will leave them with little chance to fight to get it. Most Jungle ADC or top champions can solo the dragon by this point but having 1-2 there makes it quicker and you are less likely to lose it to the other team.

I would recommend going for dragon any time it seems possible as the total gold for your team will put you at a better advantage everytime you take it down.

Baron: Rarely ever is baron taken as it spawns. Around 20-25 minutes seems pretty optimal. There's a few options here. You could bait a baron attempt (Your whole team goes to baron, making it obvious) and try to win in a teamfight, then take baron if you killed most of them. If you have wards and notice the enemy team going for baron, you can wait until their team is taking a lot of damage then go in for an easy teamfight. Most of their basic abilities should be on cooldown from using them on the baron. After you win this teamfight baron should already be heavily damaged or taken if your jungler smited it.

Baron should be attempted ASAP and if their is an easy opportunity. The buff along with the gold for your team will surely put you at an advantage to either take towers and end the game or defend if you are currently losing.