No, your mum is wrong. You're too old to have a sporting career in it but that doesn't mean you are too old to do it all. It is really a question of what you want to get out of it and what you are prepared to put in. You can enjoy yourself learning new skills and getting fit but it is a sport where you have to be prepared to put some time and effort in to improve. Some gyms that have more of a competitive focus won't take older beginners but others will have classes for beginner teens or mixed level classes for adults. You just have to do a bit of ringing around to find a suitable class. Also you could consider other gymnastics disciplines apart from artistic, tumbling and trampolining are a bit easier to get into as a late starter because they need more power and less flexibility. Don't feel that you can't do artistic though if that's what you want to do.
You're never too old! Gymnastics is for everyone, not just little girls. My daughter's coach also coaches a team at a local high school, and most of her girls there had never done gymnastics before in their lives before they joined the team. She has girls every year who walk on the team as juniors or seniors, who go from barely being able to do a cartwheel to doing standard high school level tumbling passes by the end of the season. Never think you're too old.
Also, if you just want to do tumbling, you should look into power tumbling. It's exactly what it sounds like: just tumbling.
Well, you're only going to get older so starting at 17 is better than at 90!
Wednesday was my 3rd tumbling class. I almost have my back handspring. NO you're never too late.
