When I was in my 20s, I worked on computer software to translate from one language to another. It was pretty fun stuff I thought, very hard to show you now how much fun it was as there's nothing left of it—and no pictures. Besides, it didn't really ever work.
But when my father was in his 20s, he was an electrical engineer and worked at the General Dynamics Corporation in San Diego with other engineers to build this rocket. This is a picture of it taking astronaut John Glenn, who passed away last year, into the first manned orbit by an American (1962).
I probably don't need to point out which is a cooler thing to have done in one's 20s.