Skellige's quests level requirement

Solution 1:

The main story quest on Skellige is marked as level 16, and it is not particularly difficult combat-wise. You should have no issue doing it, and it'll give you a nice bunch of XP.

There are some other quests on Skellige around level 16-20, so there should be enough to do there. You are more likely to accidentally run into enemies that far outlevel you on Skellige, so you have to be a bit more careful.

Equipment can matter a lot, so if you're having a hard time make sure you have the appropriate witcher set items for your level, or other good gear.

I did switch around Skellige and Velen/Novigrad between levels 15 and 19, there is content on both maps for these levels. I'd recommend to just try out the main quest on Skellige, it should help you level up and you'll find a few more quests for your level on the way. The main quests grant a lot of experience, more than anything else you can do.

Solution 2:

I'm finding most of the main story quests okay at level 16 but I have to say that some of the monsters you encounter, especially around some of the unknown markers, and many Witcher contracts involving monsters often seem to be too difficult. Many are at level 26 and higher. One Relict I encountered (I'll not name it to avoid spoiler) was particularly nasty, easily killing me in 2 or three swipes or killed outright with one good charge, despite my Enhanced Griffin armor, correct bombs used and appropriate oil applied. I might try my luck again if I get find a suitable decoction recipe though I later discovered it was a level 29 contract.