I think as parents of younger DCs, you can have ideas about roughly when, but it is hard to imagine them as, say, a 10yo when they're currently 4yo. It's like before you have a baby talking with confidence that you'll never do XYZ/that you'll learn a new language in Mat leave etc. Life and reality cause a re-evaluation sometimes.
For us so far (DCs are fast approaching their 7th and 9th birthdays):
have a tablet - 2yo, but only for short periods on very long journeys (ie driving to southern France). They still rarely use it now outside of the journeys.
have a phone - had planned on when starting secondary, but DCs will be going to middle school which is a fairly long bus ride away (thanks to rural location). DD1 will get one for her 9th birthday, in readiness for middle school (and the bus!) in Sept. We aren't happy about it, but needs must. A friend's DD got off at the wrong stop when she started, and thanks to her phone was able to call and let her DM know. They tracked her phone and were able to find her 2 villages over.
play date without you staying - 5/6yo as long as I know the parents and DC is happy with it.
go outside to play without supervision - 10/11? We're not there yet. I would trust DD1 to walk to the park with friends alone already, but there is a very rough and rude group of village kids who hang around there, and my biggest concern is bullying from them.
cross the road - DCs have learnt to do this in Beavers, and it is re-covered in Cubs. There has been no need to do this alone yet, but DD1 will need to get home from the bus stop in Sept.
have a sleepover - 8, provided DC is happy to.
Fully bath/shower themselves - 8 if no hair involved. DCs have long 3c curls, so will need help with that for a bit longer.
go downstairs in the morning without you getting up - 4yo
make their own breakfast - 4yo