wean - when they're interested, anytime after 4 months.
sweets - chocolate from 6 months, sweets, no idea!
ears pierced - 16! but will negotiate to 13 if pushed!
leave at home - depends on child, not before 8 ish.
walking to school - 9 (when kids move up to middle school around here)
walking to shop - dunno... probably 9ish as shop not within sight and busy road to cross.
sleepovers - family and very close friends ok once they sleep through more or less reliably (about 6 months ish here), families I don't know well, depends on child, but not before 8ish.
using sharp knife - small knife with round end and close supervision, 3 ish.
making sandwich/snack - as soon as they can reach worktop and manipulate required tools, probably around 3 ish again, dd loves trying to spread jam on her toast at 2.