I’m sure you will have no problem both growing old and watching and supporting Chelsea together man, but ever since I could remember, my grandad would take me to Millwall and I supported them until I was 6, the reason being I got the piss taken out of my Millwall shirt and came home crying, so my grandad (a die hard Millwall fan) and I watched many matches over the next few weeks and I settled on Chelsea because I was in awe of zola everytime I watched him. Obviously i doubt your boy will have that problem with a chelsea shirt lol, but I would say let them decide when they are old enough, as I couldn’t imagine being a Millwall supporter right now!