My grandparents left me a bit of money in trust. Parents got the income when I was young (nominally to help with my upbringing), I got it as a student – much appreciated it was too, a few quid spare made a huge difference to my time there – and got control of the capital at trustees (parents) discretion between 21 and 25. 30 would have been far too late IMO, if you really aren’t prepared to trust them at 25 or perhaps even 21 then don’t give them the money, simples. 30 smacks of controlling from the grave, and it may well put their life on hold re house buying/marriage etc.
Jumping a generation is well worth considering if applicable – now I’m nearly 50, I certainly won’t need anything from my parents when they pop off, but as a ~20 year old, a bit of money from my grandparents made a huge impact. And saves on IHT in the long run.