the starting age for a unit has some variability. So a unit who's normal start age is close to their old age will sometimes start old and sometimes not.
all units will eventually get old - it just might take a few thousand turns
edit: for instance, checking Edi's database, Gygia's have an old age at 300 and a normal start age of 330 - so most will start old.
a Skratti, otoh, has a normal start age of 243 and old age at 500, so you would never expect to see one old under normal situations. Some magic attacks will age a unit preternaturally fast.