Learn Finnish: Kun me ikäännymme, olemme yhä vanhempia.

You are right that yhä often means 'still'. However, when you pair yhä with a comparative adjective like vanhempia, it indicates a continuous, ongoing change. This combination perfectly translates the English structures '[adjective]-er and [adjective]-er' or 'more and more [adjective]'.

Why is the comparative adjective vanhempia in the partitive plural?

Because it is acting as a predicative (an adjective following the verb 'to be') describing a plural subject, me (we). When describing plural subjects like we, you, or they, the adjective following the verb olla must be in the partitive plural. The basic comparative is vanhempi, and its partitive plural is vanhempia.

I know the verb for 'to age' is ikääntyä. Why does the 'nt' change to 'nn' in ikäännymme?

This is standard consonant gradation. Adding the personal ending -mme closes the final syllable (-nym-me). A closed syllable triggers the weak grade of the consonant cluster, softening the strong nt into the weak nn.

The English translation says 'we get older', but the Finnish sentence uses olemme (we are). Could I use a verb for 'to become' instead?