Yes of course you can start rhythmic whenever you want!
With a ballet background you should have a fair bit of the body work ability, it all depends on your hand eye coordination and your ability to learn new skills quickly.
being flexible is a plus and strength is also just as important.
dont be put off by your age, your never too old to have a go at anything.
and if your determined and have a love for the sport i see no reason why you cant succeed. maybe im just optimistic but I’m not ever going to tell someone they cant do something, thats just asking them to prove me wrong.
maybe the other girls will have more experience having been there since a young age but if you want to learn you can of course. please have a go.
i had a student who was in her 20s when she started, she had been an artistic gymnast and transitioned over very well even at that age.
you know your abilities and if you want to try. please do. :)