There is no numerical height limit, but it will only go up so far. Try it in a shop before you buy. If they are outgrowing their current Group 1 seat, the top harness height might be roughly equivalent unless their current seat is a particularly small one (original Britax Dualfix, Cybex spin seats, Joie Tilt for instance are all quite short)
It's the best rated belt fitted 123 seat, but if you have isofix and top tether you would benefit from looking at 123 seats which use these, as they perform better. Again, check the physical max extension of harness in a shop before you buy.
If an 18kg harness is physically too small you would want to look at the seats with a harness limit of 25kg. Joie Bold for instance. These also need top tether, but there are rear facing options which are seatbelt only. Unfortunately the only seatbelt 25kg FF seat is Maxi Cosi Beryl which has more of an 18kg sized harness than a 25kg sized one (so, again, try before you buy). It's also expensive.