I feel it serves more of a purpose plot-wise. It helps to foreshadow that things change and that not everything will make sense because of age. It also helps make Anna's song, The Next Right Thing all the more powerful. I feel In Summer is pointless and doesn't really lead to anything. It's a song I skip.

funny, but ultimately useless song vs hilarious, more plot-relevant song.

This one seems more optimistic and the other song is funny, but in reality when he is older it will not make more sense. He will grow to realize how messed up the whole situation was. At least he gets to experience summer. :)

Sidenote: I think both songs are pretty much filler songs for their movies.

I thought In Summer was way funnier. I actually thought When I Am Older was more pointless than In Summer