I don't think there's a canon answer. The closest evidence we have is Bishamonten appointing Nazrin to observe if Shou was a worthy representative (SoPM). Though that only suggests Nazrin has served Bishamonten longer than Shou instead of anything directly age-related.
Speaking fanon I figured they were in the same age group. That said, if Shou were younger it would introduce more variation to the master-servant dynamic found through the series. No pair comes to mind in regards to the servant being the older of the two (actually older, and not just physically older in Sakuya's case).