We only get a very brief insight into the machine working "incorrectly" and it would seem from it that this is indeed a way to make yourself younger (and older). However, Tony himself states that the way this works is actually quite dangerous.
Tony Stark: Why the long face? Let me guess: He turned into a baby.
Steve Rogers: Among other things, yeah. What are you doing here?
Tony Stark: That's the EPR paradox. Instead of pushing Lang through time, you might've wound up pushing time through Lang. It's tricky. Dangerous. Somebody shoulda cautioned you against it.
It seems that whilst they could change Scott's age they could also just as easily erased Scott from time by ageing him too much. It appears to be a very dangerous thing and so whilst possible to use I imagine most would turn against using it for that reason.
Also remember that Tony is the one that got the time machine working and with him gone it would seem that they might not be able to get a working version of the ageing machine up and running. Remember Bruce even says himself it is outside his area of expertise so it seems lucky he got as far as he did.
Bruce Banner: Right. The Hulk time travel do-over? Guys, it's outside my area of expertise.
Lastly, with regards to Steve himself it seems very doubtful he would try it. He's lived his life now and has passed the mantle on to Sam. Avengers: Endgame marks the end of his journey culminating in him stopping being Captain America altogether.
(Old) Steve: Well, after I put the stones back, I thought... Maybe... I'll try some of that life Tony was telling me to get.
Sam: How did that work out for you?
(Old) Steve: It was beautiful.
Sam: I'm happy for you. Truly.
(Old) Steve: Thank you.
Sam: Only thing bumming me out is the fact I have to live in a world without Captain America.
(Old) Steve: Oh... That reminds me... Try it on. How's it feel?
Sam: Like it's someone else's.
(Old) Steve: It isn't.