Can unaccompanied teens get through customs?

My two daughters, ages 16 and 18, will be flying into Narita and then taking the train to meet me at my hotel. Do they need any documentation (other than a passport) to get through customs or get tickets for the train? I could meet them at the airport if I had to but it's an hour away from where I'll be staying. There is probably a website with rules about this but my feeble Google searches only gave me customs in the sense of traditions/etiquette. I'd be happy to hear any general words of advice too! They are not nervous at all but I am!

One is an adult. So I see no issue.

Just be sure they have the full correct address and phone number of the place they are staying, so they can fill out the immigration/customs form, that is handed to them by the flight attendant prior to landing. Otherwise I don't see any problems.

Your going to the airport wouldn't achieve anything. You are not allowed air-side and can only meet them after they have cleared immigration and customs.

And while some countries like families to approach immigration as a group, that does not apply in my experience and observations for Japan - for little kids maybe. Your daughters would approach the immigration officer separately/individually for finger prints, photo and processing.

Customs (after baggage claim) are also likely to be quite uninterested and their questions are normally about luggage and contents anyway, sometimes about the stay in Japan.

And the real hurdle is actually the airlines - they are not going to let someone board if they were not going to be allowed into the country (Japan) - they suffer to much in terms of fines and costs. And 16 isn't particularly young to be flying solo and your 18-yo isn't considered a minor in many countries (although adulthood in Japan is officially 20). I know people younger than that who have traveled solo between Japan and the USA (a 15-yo I know as an example).