Still think of Walker as young! Regardless he's about as much of a leader as Theo Walcott. Otamendi will likely be the one to make way for Laporte, Guardiola loves John Stones. Delph is not really relevant to the point I'm making.
Still, Ederson, Laporte, Stones and Mendy will be a very young back 5 moving forward.
They need a Hummels, Ramos, Boateng, Pique, Godin, Thiago Silva... someone of that profile, and with their money they probably could get it too.