Well, Shrek and Fiona are about the same age. If we are to consider Shrek the Musical to be Canon, Shrek was sent away at the age of 7 to live on his own, and so was Fiona. Fiona spent at least 8423 days in the tower, which is approximately 23 years, so if we add the original 7 years, that gives us Shrek's age of about 30. Every Shrek film seems to take place right after the last one, with the exception of Shrek 4, which takes place one year after Shrek has kids in Shrek 3, which means that Shrek is 31 now.
To Summarise, Shrek is probably in his Early 30s.