I know that all creatures level up with you in this game, so going from that point I can go to Shivering Isles at lvl 1. But I heard that the quest rewards gets better and better if you visit the place at higher level.
Tell me your opinion, which level you think its optimal for entering Shivering Isles?

You get the best version of all the rewards if you are level 30 or higher. So if that's what you're after level 30 is best.

I specifically waited until I was level 30 so that when I got the unique weapons and armor they were the best they could be. If you go in at level one you get these unique weapons (which are the strongest in the game), but instead of them being great they will svck because they are leveled as well. My advice wait until higher levels to enter the shivering isles.

From this it sounds like you want the best quest rewards, so as said above, level 30 is optimal.

My character tends to go in very early and bounce back and forth between SI and Cyrodiil. My only advice would be is to look carefully at the quest rewards. Many people like them, but there is not a single leveled quest reward that interests my character; the freedom of not caring about that is wonderful. I much prefer gear and weaponry that is self-enchanted.

The other aspect of SI is the amber/madness gear. That is neat and kind of unique in that when you have it made, it is made at your current level. That means you can go back later and have better stuff made. Amber/madness tops out with Perfect at level 23.

Well its lovely in the isles right now perfect time for a visit

Thanks for the replays guys, I will wait then.. I will progress a bit in Cyrodiil first. However I dont think I will be able to wait for 30 lvl to enter Shivering Isles, because as I can remember that place was really fun.. Cant wait to start the quests there. Again, thanks for the help and have a lovely evening.

In my opinion the best time to travelto shivering isles is when your level is 10 or 15 because the random loot gets better

Theres some amazing loot in The Shivering Isles when you reach level 30, so I agree that you should wait till then.