What are the demerits of pushing early in League of Legends?

There are several reasons for that.

• Towers are particularly strong, early game, in League of Legends

Even if you want to push, you won't actually be able to destroy a tower during the early game, and it can kill you quite fast. So while it seems intuitive to push when you are at advantage, it's a strong limit preventing you to push further, even if the lane goes good for you. This leads to the next point...

• Positioning

When you push, the distance to your tower (and so, your safe place) is increasing, while it's decreasing for the enemy. As such, it will be complicated to actually damage them and even more to kill them. And at the same time, you are more exposed and have a greater distance to walk if things go wrong.

• Minding the jungler

You might not have this situation in early levels, but the current metagame is such that you normally have one player in each team roaming in the jungle, and ganking lanes.

As such, by pushing your lane, you expose yourself to the enemy jungler while the enemy is protected from your own jungler, safe at their tower.

• Optimizing your farm

In the current metagame (at least for western countries, it's a bit different when you see korean players, for example), there is a phase of heavy farming at the beginning of the game. To optimize this farm, you need to be safe, and have the minions accessible.

That's why pushing too hard, and killing the enemy tower too early can be a problem. In early game, the enemy tower helps you with farming, because it limits the distance where your creeps are going. Since there is no denying in LoL, it's actually the only way to have a sure limit for them.

Without the tower, your farm area is easily pushed farther in the enemy territory, which doubles the risks from the two previous points.

The only advantage I can think of pushing your lane is to force the enemy at their tower. Farming under the tower is complicated, as the tower will take a lot of the last hits on creeps. By pushing the enemy under their tower, you can deny them a fair amount of gold.

But it remains risky, and it will be enough for their jungler (or mid-laner) to come to your lane, to force you to retreat (unless you really have such advantage that you can take them 1 vs 2 or 2 vs 3, depending on the lane). So even if it looks like you are at an advantage, you might actually be offering them an easy kill or two. So careful with that, as the matchmaking level increases.

When to Push

In most cases you will not want to push the lane since this makes you open to ganks, however there are some situations where it does make sense:

• You notice the enemy team has gone for an objective (they are all pushing another lane, doing dragon) and you will not be able to reach the fight in time. You would be better off pushing your lane (so that the enemy champion that would have been in the lane loses experience, and if you push hard enough, you might even be able to get the tower).
• When you have your summoners up and perhaps a built-in escape. If you push the lane on your enemy, they are more likely to miss minion kills at tower (it is harder to farm at tower effectively). This makes you susceptible to ganks, however if you feel comfortable that even if you were ganked you would be able to evade, then it should be okay.
• When the enemy team is going to be very ‘late’ to lane. This is common at level one. Sometimes the enemy champions from top or bottom lane will stay to protect their jungler, by doing so they will also be late to the laning phase. If you are able to push the lane, they could miss one or two waves of minions. This would give you a significant advantage in lane and well worth the push (even if you are more open to ganks).

When not to Push

• You know that the enemy jungler has finished their jungle route (After 3:50/4:00 on the game clock) and is looking to gank the lane
• When you don’t know where the jungler is (you do not have map awareness)
• You don’t have your summoner spells available (they are on cooldown)
• When your enemy jungler wants to gank the lane and you want the lane to push to your tower.

When to Keep Lane Even

This means that you are trying to keep the creep line about even with your opponent, the minion line will not push to either tower, you will simply keep equidistant between the two towers.

• When you think that the enemy Jungler may be on your side of the jungle (the mid lane splits those sides (top and bottom)).
• When your allied Jungler has no plan to gank the lane anytime soon.
• When your opponent is hitting the creeps more than once (they are pushing the lane on you) You will want to match the amount of damage they are doing to the creep line. This might mean you would use abilities to clear minion waves if your opponent is doing the same. Keep the same pace so that the minion lane stays in the middle.

Source.