Both my girls where in them after outgrowing their 5 point harnesses, they would escape a normal seatbelt too easily. With ds the new boosters with higher shoulder slots for the 5 point harness he is still in it at almost 4, he should still be in it for a while as he still has one more slot to go much prefer that to the H harness.
This is the bit I don't understand at all - nearly all convertible boosters I have seen have harness slots higher than the change to booster mark - but when I asked Infasecure this was the reply

Me: Thanks, can you clear something up on the convertible booster is the change to booster just a guide or does it mean that once past the range of that mark you should definitely be changing to booster mode (irrelevant of age/weight).
2 hrs · Like

InfaSecure Hey Victoria! The marker is a guide, but we do recommend that it's followed. Our convertible booster 'change to booster' markers have gone up, recently, however, and are now at 45cm. Hope that helps!
So what does that really mean - which is safer? (I suspect their answer would be buy a new convertible booster with the 45cm marker!). Also do I need to ask all the manufacturers for each individual seat the same question. Surely it should say somewhere what the 'change to booster' really means?

It would be great if there was a site you could put your little ones age, weight and height dimensions into and be given a range of seats and including how long you could use each. They say weight is not important now, but still many of the seats are only tested within certain weight ranges so surely can not be entirely discounted?