Aside from the nearly crippling arthritis, bad eyesight, and hairline that receded faster than the French in 1939, I'd say having the first sex talk with the oldest boy. I made sure to not make it as awkward as mine was, but then again I didn't pass down specific tips for picking up and fucking a hooker bareback either.
He's within a few inches of me and hasn't hit his growth spurt yet. I feel old some days.