It was previously announced that Nintendo Accounts would be open for registration as of February 17. Yet the roll-out has arrived early, giving opportunistic Nintendo fans leverage to claim they're ahead of the pack.
The sign-up process is very, very simple (unusually so, for Nintendo). Connecting to your existing NNID will pull all your information — name, date of birth, email address — for the registration of a new Nintendo Account. It takes seconds to do. Alternatively, you can use your social media accounts to accomplish the same effect.
There's no need to rush to claim an ID; it's not like your Nintendo Network ID, which cannot be assumed by another user. The name associated with your Nintendo Account is freely editable, which is a plus for anyone with an aversion to permanence. Not sure what they were thinking with the 10-character limit, though.
Again, no rush, but by registering early, you'll earn yourself an unknown bonus in Miitomo. Want to wager it's an accessory?