There were different rules of comic book time when Weisinger was editing Superman than when Schwartz (and others) were editing Superman. Mort essentially had it that Superman became active in 1938, before that he was Superboy. How you figure that out is your problem, but comic book time and real time didn't need to be the same thing. So characters just remained stuck at the same age even though time was passing all around them. However, with Supergirl it was different. She was perhaps 14 when she arrived on Earth (1959) and she acted like a little girl. But relative to Superman, Lois and Jimmy, Linda Lee aged rapidly and entered college in 1964--which means she was ageing almost in real time, but then her age slowed down after that, yet not completely.
When Schwartz and the other editors took over in 1970, it was stated that the rules of time had changed and Superman would now stay at a permanent age of 29, with the Superboy stories happening about 15 years before the Superman stories (more like 13 years--or even 12 years when Mike Grell was drawing Superboy, since he looked older by then). Supergirl got out of college and bounced between a few different jobs, so she was probably in her mid-twenties at that time. But then in the early 1980s, after Supergirl got her own new title, she was de-aged to a younger age, before dying in the Crisis.