One was always gentle and one is 10 and still can’t do it.

This was our experience too. It was somewhat subtle, but my older child - while not aggressive - had a way of being around our dog that made her uncomfortable. Despite lots of coaching, he'd do things like drape himself on her back, swing around toys by her face, etc. The dog really did not like him, but was totally fine with his little brother. Kids were 7 and 5 when we got the dog. It was weird, since the kid in question who caused the problems was otherwise pretty calm and collected.

Now at age 11 older kid and dog have finally formed a bond.

It depends on the pet too though! Both kids and the dog we had previous did just fine together. It was the combo of older kids personality and new pet's personality that was less than ideal (not awful - just required a lot of monitoring).

I would say 3.