Help -need inf. on 258th ???

I came accross an old photo of a cousin who according to the inf. on the back of the photo states the following:
"Telephone Detail Bat. C - 258 ??? Ft Ontario, New York
dated 1931 can any one give me inf. on this outfit.
Do not understand how he could be in the Army he was only 16-17 years old. (guess in lied about his age)

Also are there any records for those who tried to reg. for the WW II draft and were classified 4-F

Thank you