DS1 didn't acknowledge his legs until he was about 15m: if you held him up, he'd lift his feet off the floor. No weight bearing. Rolled briefly at 4m, and then never again. So at 15m, if he was lying on his back, that was where he was; if he was sitting and someone bumped into him, he'd fall over and couldn't sit up again. Bum-shuffled happily around (pulling himself with his arms - no leg action).
Sent to urgent paeds as hosp, as GP and HV fussed (at about 13m), who said yes, bit odd that he won't weight bear, but no signs that anything hurt him/was out of place, and to come back if he wasn't walking at 18m.
As it happened, he wasn't, but he was cruising by then, and at just short of 19m walked across the playground as if he'd done it forever.
Is now 7.5, one of the fastest runners in his class, climbs, rushes about.
Has always been an amazing talker - very articulate, big vocab. So I reckon some children just do one thing at a time. Not walking at 14/15 months seems just fine to me.