The moment Harry turns 25, he will inherit the money left to him by his mother. However he will have access only to
interests (more than £300,000 a year) until he is 30, after which he will have full access to his inheritance. This means that while Harry can spend only the interests, he can make provisions concerning the future of the money (for example, write a will and designate who would receive his fortune should something happen to him).
I would imagine the will also includes special clauses which could, in theory, give him access to the main sum before his 30th birthday, such as unforeseen situations or urgent / vital circumstances: although this would be a more or less standard thing to do, we cannot be certain Diana’s will includes any such clause.
Diana’s will follows the usual pattern in cases like this: it makes sure that the beneficiary has sufficient funds to maintain a comfortable lifestyle (from interests), while at the same time guarantees the inheritance will not be wasted away by hot-headed youths before they are really in need of it.
Both William and Harry’s money had been invested, so I think Harry is set to inherit considerably more than £6.5 million.
Please, note that the conditions I’ve written about are the common practice: Diana’s testament might have contained special clauses and conditions we know nothing about.