You may be doing your wallet a favour but, may not be doing her a favour in the long run, as she would have missed out on 6 years NCB, which would likely impact her just as much at 25.
Not so sure about that. Missing out on NCB will be offset by a much lower cost of getting insurance for the first time by being over 25. My daughter recently got her first policy after being on my insurance for a few years and paid just over £700 for fully comp. She is 25 and has held a full licence for 6 years but had no NCB.