Hello dear 

There's no age limit for joining under graduate courses and then completing it , one can do it at 20 or 30 or even at 40 as there's no age limit for completing graduation.  But there are special under graduate  courses like medical in which mbbs which is under graduate courses is not allowed after 30 year of age and also to appear in exams like jee there's age limit but you can do engineering at any age from private colleges .

Thankyou 

Feel free to ask your further queries if any