Zelensky was 36 years old in 2014.
Here is a 2014 NBC article about the newly formed military draft that year.
May 1, 2014
“Acting President Oleksandr Turchynov issued a decree on the presidential website Thursday saying Ukraine will conduct a draft for active military service in Ukrainian Armed Forces in order to “to increase the state defense potential.”
The decree says: “Ukrainian male citizens who are physically qualified for military service, over 18 years old and older, but who have not reached the age of 25, and who have no right for exemption from military service” will be conscripted.
The statement says such measures are being taken because of the “deteriorating situation in the east and south of Ukraine … activities of illegally formed armed pro-Russian groups”
https://www.nbcnews.com/storyline/ukraine-crisis/ukraine-enacts-compulsory-military-draft-n94906
Here is the relevant legislative decree from March 2014. No mention of any age limitations associated with this partial mobilization order are mentioned. Conscription to the AFU took place after the mobilization effort failed. Both were very unpopular in Ukraine.
PRESIDENTIAL DECREE OF UKRAINE of March 17, 2014 No. 303/2014