In the pilot episode Dean is 26. Given that the show is in season 15, he would be 41. However, the show conveniently forgets that there was a year ago between seasons 5 and 6, as well as 7 and 8, meaning the show should technically be in 2024, making Dean 43.

In season 9 of Supernatural, Dean Winchester would be around 35-36 years old. Dean was 26 in the first season (2005).