It isn't possible to find the exact creation date of your runescape account. Unless you remember.
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First go to the RuneScape Homepage and click Account Management under Account. Under Account Setting you will find Change password. Then Log in and change it.
It is not possible to delete a RuneScape account.
That's the sweet part of runescape by adding that now it would be harder to hack your account. But otherwise you log in in as your original account to get on to runescape.
There is currently no option to delete your RuneScape account.
Look about the site and find out.