In the anime, Kenshin appears to be about 17 (at least to my western, Caucasian eyes). Consider that the revolution was 10 years in the past, and it must have taken years during the war for him to develop his skills and reputation as Battosai the Man-Slayer. At a minimum, I think he's got to be 31, and that is if he got started in the revolution while still a mid-teen.
Kenshin was born June 20th, 1849. Sold into slavery at age 6 (1855) Trained until his departure at age 14 (1864) when he began his Hitokiri days, obtained the first vertical Half of his trademark scar. By 1868 he abandoned his assassin role for guerilla swordsman duty protecting Imperial officials. 1869 Boshin War Ends (Kenshin Age 20) Proliferates his wandering days for nearly a decade til he arrives in Tokyo in early 1878 (Age 28) OVA: reflections: 5 years later, dies at age 33 of unknown disease (Presumably either Leprosy or Meningococcal disease)