I'm 26 and keep getting better.
First k-fed when I was 18, first backflip at 22, first 5 in the pipe at 23, tried my first dub back this year.
20 is super young, people love to hammer on about how old they feel in their twenties, but it's mostly in your head. I have some nagging issues but nothing that hasn't been overcome by the stoke to learn a new trick.
As you get older you become more risk-aware, which can slow you down. On the other hand you gain from experience a better understanding of how to do certain tricks and the steps you take to learn them. When I was 15 I had no idea what the proper way to spin a 360 was, when I was 16 I was clueless about how to back flip (I just found a quarter pipe type feature and hucked). Now I have the knowledge of how to do most tricks, the hard part is finding a good/safe feature to try them on.