When Goku wishes back Earths population he made the stipulation that everyone had to come back with a good heart. And because the Androids are originally humans 17’s heart was affected by the wish. 18 became good when Krillin tries to wish her into a human but Shenron explained that it’s pointless.

Can Android 18 get old?

re: Do Android 17/18 age? No, they don’t. If they do, it is a very SLOWN down aging process if anything… just getting started!

Is Android 18 stronger than Frieza?

It’s not only that they were as strong, but they don’t run out of Ki and Stamina so they can keep fighting beyond exhaustion of a normal flesh being. 17 and 18 by my calculations were about 2 times as strong as Freeza.