BFing to sleep - experiences on how/when your baby learnt to self soothe?

[Warmseabreeze]

Currently at home DD2 21wks will only fall asleep if BF, if in the car or pram she will fall asleep if tired. I just wondered what experiences others have had with this, and how or when your LO learnt to fall asleep off the boob.

DD - at 10 weeks found thumb and could fall asleep even if exhausted. she stopped recently age 6.

DS1 at 6 months stopped feeding, and bought him comforter. he still sucks it age 4, and can't sleep without.
comfoter

DS2 self settled from birth. Goes in cot awake, and then falls asleep no problem he must know he is a 3rd child.