Fun has no age limit.
Bloody hell man, building Lego with yout mate would drive me bonkers; I'm methodical.
This one will be maybe 12-16 hours all up. I don't go fast. There's no electrics on this one, not much in the way of gearboxes and all. I've built some with 4000+ pieces. This one is 1500 or so. One with way more pieces and complex gearboxes can take up to 24-26 hours.