Tom has always wanted to be a father but has not yet found an acceptable partner. he is now 35 years old, healthy and beginning to be concerned about how much longer he will remain fertile. when he asks his doctor about this, his doctor tells him he will probably be capable of fathering a child:

Technically, men can have children until the end of their lives, and people sometimes hear of 80 - year-old fathers. 

However, the male sperm starts to decline in number and quality from the age of 40 and decreases dramatically after the age of 55. 
So Tom can relax a bit, but when he does find a suitable partner, he should insists on pre-natal screenings to make sure that his child is healthy.