Parents get too hung up on this shit (I've been guilty of it also)
There isn't a "right age" for anything. Tying shoes, riding a bike, ditching a blanky, using a stroller, breastfeeding, etc. Every kid is different and quite often you're just causing extra anxiety or tension trying to force your kid into something they're not ready for.
That being said, there is an upper limit to this shit where it becomes detrimental, but I don't think 6 years old in a stroller is hitting that yet.