Is 15 too young to do an overnight on my own?

Too many unknowns to answer that question. I would suggest something closer to home as a first tramp. The problem with Tongariro is that it can have severe weather any time it likes and that weather can kill people

@geeves, fair comment, but @giraffe-soup, I say go for it! The most important aspect on your plan is the weather, so pick a window of settled weather and bust a move. You're familiar with the track and terrain; you've done the tramp and camped several times; you've got solid experience and equipment. Edit 5 minutes after posting: Your mum has the final say! I am stating my personal opinion here and don't want to get between you and your mum.

Identify the risks - like weather - and ensure you have a plan for them (like moving your trip dates). Carry a PLB. Talk through your plans and risk mitigation strategies with your parents Under 16 they are your guardians and have legal responsibility so they need to be convinced. However, if you manage the risks, develop independence, show initiative.... what's not to like? I was roaming the Tararuas hunting at that age and loved every minute of it. Good luck!

I don't think age alone disqualifies anyone from doing a solo overnight tramp. As mentioned by the above posters, your personal skills, experience and knowledge of the area/terrain as well as equipment (and ability to properly use it) are more important. So is having clear communication about your intentions, back up plans and what you would do in an emergency and sticking to these as much as possible. Having said that, it is definitely an endeavour you need your parents' buy in for. No matter how competent you are I just don't see how it goes well if you somehow decided to do this against their wishes. If they are dead set against it, then you need to accept that. Taking the L on this one but use it as a chance to show your maturity and potentially establish what they feel is required for you to do the trip in the future. That way, when the topic comes up again you can present a strong case.

For what it is worth two of us did a very similar trip in the May school holidays when I was 13 - way back in 1968. We had quite the adventure and one close shave with exposure, but I remember those trips decades later like they were yesterday! It was the start of a lifetime of wonderful experiences and I would have traded it for nothing. I fully support your intention here and would love to hear back from your here after the trip. Having said that the world has changed. Sadly we wrap children in cotton wool far too much, leaving them unable to face real risk and challenge. It serves them very poorly in adult life. Yet I can understand your Mum's hesitancy - and if she says no then that should be the final word. As others have said, she has legal obligations to be concerned about. If you are planned and prepared I see no good reason that your age alone should be a problem. Maybe you could show this thread to your Mum - it might give her some assurance that your dream is not at all crazy.

My brother and his mate did a 3 day trip up the Otehake and down the Edwards when they were 13 or 14 years old by themselves. Laurie had done the same trip with our Dad and brother(s) the year before and they had been held up by rising rivers in the Otehake. I think his mate's parents had no idea what the boys were undertaking. My brother Brian used to bike on his own to Birdlings Flat from Chch, put up his pup-tent and stayed the night on more than one occasion. Of course his older sisters asked Dad if they could copy him and were told no - Prepubertal lassies. Brian was 9 years old.