Hi there,
I agree, there is no age limit in reality. In your mind if you think around 30 is too old then probably for you, it is.
I guess by using the word 'star' that would narrow it down quite a lot but if the musician is doing something they are satisfied with and it happens to be a sound that folks find interesting.... well then, no limits.
What you are seeing and calling a 'star' in most cases is in reality a commercial product who's potential to make large amounts of money as been spotted by either a record company or an individual so in this sense, I guess young is more marketable than someone in their 40's maybe.
Go for it, you still have about 8 years...
Good luck, but always remember to do what you want and not what you think someone else might want...
Gordon.