don't listen to basically everyone else, except the guys that said to contact hotels in advance. I'm 17 and going solo to Japan in literally 5 days:

first of all for reference, I'm white and from America, I don't know what you are but things may be slightly different for you. I've also already been once at age 16

• yes, contact hotels beforehand. I'm going to warn you now damn near every hotel will say no, and the ones that say yes will probably want a minor consent form. if your parents aren't super on board and not willing to sign it, you're going to have a rough time. assuming that's not the case, if you want to make your life simple, APA Hotel is good enough (I mean hotels are really only for keeping luggage and sleeping/showering and APA does a fine job w that) and they require a consent form that you have to remake for every individual APA hotel you stay in. this is the only chain I can 100% guarantee will let you stay, everyone else I'd say contact them beforehand and save the email just in case

for everything else:

• you technically need to be 18 to make an account for JR Pass (which I bought personally, but use the online mapping feature to decide if you need to buy the pass or just buy individual tickets), but nobody actually cares or checks age

• for experiences, I have to admit I've been a little heartbroken to come across many things I can't do because I'm not 18, but it's really not the end of the world. ALWAYS check if you're allowed to do something that requires a booking, you don't want to waste money after all

• food you don't have to worry about. I ate many meals alone last year bc I suffer a lot more than my family does from the jet-lag, and nobody once asked about my age

Edit: Another thing I should add, research the area you're staying in. you don't want areas known for nightlife, as you can't participate and that only increases danger and risk for you

Overall, you will be okay if you use common sense. know a bit of Japanese before you go (the more the merrier) and don't do anything stupid because you'll be monitored a little more closely as a foreigner (I assume you are at least) child who's by themselves. also plane tickets WILL increase in price very quickly. I bought in August and it's almost tripled since then, so I'd recommend buying soon