Well, I think another piece of the answer to "why do RCs customarily do it that way?" (as opposed to "is that the best way to do it?" is that for centuries before Vatican II, laypeople received only the host, not from the chalice, so giving Communion to infants would have involved "starting solids" as it were (i.e., you don't want a baby to gag/choke). The timing of First Communion/Confirmation stayed about the same after Vatican II, though I believe there have been some efforts to get Confirmations pushed earlier.

I am curious about the timing-of-Confirmation thing too. It's not something I know a lot about.