Marcus Allen -- I'M A DADDY ... 53-Year-Old's GF Gives Birth to Baby Boy

Marcus Allen's still got it ... 'cause the 53-year-old NFL legend just became a daddy, TMZ Sports has learned.

We know ... Allen's girlfriend Lauren Hunter gave birth to Marcus' son, Drake Connor Allen, on January 5th.

Sources close to the couple tell us ... the two met several years ago on MySpace (back when MySpace was a thing) and they hit it off.

We're told Lauren has two children from a previous marriage -- and it appears the child is Marcus' first.

FYI -- Lauren is a fitness model ... and she's SMOKIN' HOT.

Nice work, Mr. Allen.