Lets start with a character we know their end age, of course, Edward Elric.
At the end he is 16 (brotherhood) and born in 1899. 1899 + 16 = 1915. If youre like me, dont worry, Trisha Elric died 1904. So we have an ending year, 1915. We just so happen to have Riza's birth year, 1889. 1915-1889 gets 26. Riza Hawkeye is 26 years old.