That's 1,951 years ago, boy (3)

I believe the answer is:
lad
'boy' is the definition.
(informal term for boy)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for lad that I've seen before include "(Housman's Shropshire) boy" , "A young chap into ballads" , "Scottish lover" , "Shaver" , "Chap, fellow" .)
