We had some (pretty rubbish but you could get off the ground) trees in our garden, and we'd spend hours up them as kids. DH also had great trees in and around his garden, and also spent a lot of time up them.
We live in the city now, and no climbing trees in the garden (pocket hankerchief). But there are a line of large leylandii spread apart, that the inner trunks are like steps of stairs so the local kids are up them from almost as soon as they can toddle. They used to be 12' tall when DD was going up them first, but got halved a couple of years back so now about 6.5' (I can grab a child at the top from a full stretch). DD has been going up that since she was about 2.5, if not younger.
There is also one other main climbing tree - where you have to get up a smooth trunk to 4' up. So they tend to be about 6-7 before they manage that one, or else the older kids help them up. DD is now 8, and swings out of that (and has done for quite some time), and has moved on to getting onto the wall about 3' away from it too (
) all the time. DD and her buddy are the youngest 2 of the main group on the street (there is a 4 year old and a 2 year old but the older ones are too old for them - but not quite far enough apart not to tolerate the 7/8 year olds).
SIL was not impressed though with DD trying to introduce her DCousins to the trees when they visited a couple of months ago - they range from 3-6, and are great kids but very sheltered (despite effectively living in the garden where there are still climbable trees from DH's youth!).
Like others, I am very happy when DD is swinging out of those trees or running around the green, as it means she's not curled up watching tv or ensconsed in Minecraft or similar! Some screen time is ok, but fresh air is great too.