if spike was 16 in the 1984 cartoon,that means in the year 2005,spike would have been 36 years old & 37 in season 3. spike should have been 39 years old in season 4.

spike sure didn't look that old (36 & 37 years old) in the TF movie &
in season 3. in season 4 it's widely specualted that at least 2 years
went by from season 3 to season 4. season 4 seems like its set in the
2008 year. so in 2008 spike should have been 39 years old.

at best spike looked like he was 27-28 years old in the 1986 TF movie
that was set in the year 2005.

at best spike seemed like he was 28-29 years in season 3.

at best spike didn't appear to age anything from season 3 to season 4.
it was apparent a few years went by but spike just didn't age.
so spike appeared to be 28-29 years old in season 4.