When are kids old enough to play outside alone?

When I was three I was allowed to drive my tractor up and down the front driveway of where we lived (St Donagh's Rd, Donaghmede.. very posh area of Dublin for those not au fait with Dublin) as where my neighbour's kids my age. The gate was locked and I would regularly stand at the gate saying hello to strangers until the acknowledged my existence. I can remember running inside chuffed when someone did say hello to me. Not that I think of it, life has changed all that much for me tbh.