As early as possible.
This will give you years to discuss it, what opportunities it gives her, how not to waste it and also, hopefully, to use some of her life chances to help others.
I second this…my DC learnt that they were in my cousin’s will at 14.5 and 17. It was a year before the money (almost 6 figures) was distributed and there was no minimum age stated in the will, so DS got it immediately, DD’s was entrusted to us to manage (instead of solicitors and with her permission) for the last 2 yrs until she turned 18.
We started casual discussions when they received the letters informing them that they were in the will and ramped them up a few weeks later when a breakdown of the deceased’s assets indicated how much they might inherit.
We pointed out how lucky they were to have a nest egg, how much houses cost, how if they frittered it away, there would be no replacement. We also advised them not to discuss the money with their friends.
We already had S&S ISAs for them, and worked with them to drip feed the money into those monthly. If they want to blow it all, they can access the money, but not as easily as if it was in a high street account.
DS, now 23, recently turned our words back on us, emphasising that he realised how very very lucky they both are to have the money. I think it has actually helped them to save generally, as they can see the figures growing each month.
I would wait until she needs it.
That is not necessarily good advise, depending on what age she formally inherits. It is hers from that age onwards, Op cannot withhold it.