Yeah Carvalho 26 and Terry 24, Rio 28 Vidic 25, Per 27 Kos 26, Puyol 30 Pique 21, Maldini 34 Nesta 26...these partnerships are usually similar age but older or one is significantly older than the other.
You could play a 19 year old next to a 22 year old in Ligue 1 or the Eredivisie but if you do that in the PL you're not gonna get anywhere near top 4.
I think if he comes in we will stick to the back 3 with Luiz in between Saliba and Gabriel. I think on paper:
Saliba--Luiz--Gabriel
is superbly balanced, and would keep loads of clean sheets.