Really, it depends. Age isn't as much a factor as your needs changing as you age. Want to start a family? You need a consistent income that will allow your family to do more than subsist. I could probably try and play in Challengers through my 30s, but id probably have to maintain a part time job, and rely on family or SOs to do it.
Pretty sure most pros who get their start at an older age (I.e. glowiing/pwnalone/roej in cs) have already been playing in lower circuits for some time.
End of the day, if you have the skills, comms, and temperament, you can do it at any age.