Okay, I'm 17, about 5"3 and weigh just over 12stone.
Yes, I know I'm overweight.
I was a regular gymnast in my younger days (age 5-9) but stopped when my parents couldn't get me to the meetings anymore (too far to travel and not on convenient days)
Since then I've not done any regular sport and have a few problems, such as joint pains.. although I guess that's just because of the weight gain.

Am I too old, and out of practice to start again?
It's something I enjoyed, and one of the sports I actually enjoyed. (I was never one for football/netball/hockey etc.)

You should be able to get back into sports
You are only 17 and you did have a past with it so it isn't like it would be something new and hard to get use to. Sort of like riding a bike you never forget.

Nothing ever too late, if you feel you can do it, go for it