Why would I lose all my gymnastics skills as I got older?

I have been doing gymnastics for 10 years and I competed when I was 12, 13, and 14, 15 and 16 and when I was at that ages, I was the best at it. I am now 18 years old and I haven't done gymnastics in a year and I don't have the stamina as I used to and I'm starting to lose all my gymnastics skills. How come?
Why would I lose all my gymnastics skills as I got older?
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