17 and planning ten days in Portugal alone — what actually blocks a minor from booking?

Did Spain and Portugal at 17 and the honest answer is that policies vary per property, not per country. Plenty of hostels are hard 18+, some take 16 and 17 year olds with a signed parental consent form, and a few will only put you in a private room rather than a dorm. The booking platform will happily take your money either way — it's the front desk that enforces it, and 'the website let me book' carries no weight at 11pm. Email each place before you pay, ask them to confirm in writing that they accept guests under 18, and keep the reply on your phone.

€900 for ten days in Portugal is workable but not luxurious, and it depends almost entirely on how late you book transport. Beds in Lisbon and Porto were my biggest fixed cost, food was genuinely cheap if you eat where the builders eat at lunch, and the thing that quietly ate my budget was buying intercity train tickets two days out instead of two weeks out. Book Lisbon–Porto early and you'll free up a couple of nights' accommodation.

My mistake was assuming nobody would ask. At a land crossing at 17 I got pulled aside and questioned for the better part of an hour about who knew where I was, purely because I had nothing on paper. Since then I carry a signed letter from a parent with their phone number and a copy of their ID, and nobody has asked again. Rules differ by country and by whether you're crossing an external border, so check with the relevant embassy rather than trusting a travel blog.

I'd skip the emailing-hostels fight entirely and book small guesthouses and pensões with private rooms — barely more expensive off-season, no age argument, and you actually sleep. The thing I'd spend an evening on instead is travel insurance: several policies I've read exclude unaccompanied travellers under 18 outright or require a named adult. Ring the insurer and ask that specific question rather than assuming, because that's the one that matters if something goes wrong.

Private rooms, written confirmation, consent letter, insurance that knows your age. Everything else is the same trip anyone else takes.