Simply put, there is no age limit for someone considering doing a PhD. Indeed, on the opposite end of the scale, even the definition of the minimum age at which someone can start a Ph.D. is not really well defined.  

One of the youngest PhD graduates in recent times is thought to be Kim Ung-Yong who is a South Korean professor who purportedly earned a PhD in civil engineering at the age of 15.
For the vast majority, however, the practical considerations of progressing through the different stages of education (i.e., high school, undergraduate degree, a Master’s degree, etc.) means that most won’t start their Ph.D. projects until they’re at least in their early to mid ’20s; in the UK, for example, the average age for a Ph.D. graduate is between 26 and 27 years old.