There's no age that you should stop wearing Jeans.
Jeans are made for all ages and I've never heard of anyone stopping wearing Jeans by a certain age.
My Grandpa is 90 years old and he's been wearing Jeans all his life and will continue to do so until he dies.
You can wear Jeans or any type of clothes that you want no matter your age.
I'm almost 40 and wear Jeans as they are very comfortable to me and I will wear them for the rest of my life.
However sometimes I wear sweatpants when I don't want to wear the Jeans but other times I wear Jeans.
In the summertime though I will wear shorts as it's cooler that way as I can't stand being too hot in Jeans in the summer but in the spring time weather and winter I mostly wear Jeans.