For Amsterdam, I would suggest Cocomama - it is a boutique hostel - all their dorm rooms are unisex, although they do have some private rooms and some small 2-bed dorm rooms that can be booked as a private if you so choose - or just book 1 bed and you've got yourself a relatively quiet room to share with just one other hosteler. If you don't mind being in the minority it is really not a big deal to be over 40 and stay in a hostel - although you should very carefully read reviews before you select one. Yes, the majority of hostelers seems to fall in the 18 to 35 demographic, but at any hostel I've ever stayed at, probably about 5-10% are over 40. I will say in my experience, if you want to find more of the older hostelers, then some of the swankier "boutique"/"flash packer" hostels that cost a bit more tend to attract more older guests who still want budget, but can afford a little bit more than your average, cash-strapped college student.
While "Let's Go" guides might list more hostels than the typical Rick Steves' guidebook, it's still very much geared towards college aged travelers. I would suggest looking at hostels and budget accommodations (hotels, guest houses, B&Bs) listed in Rough Guide or just go onto a website like Hostelworld.com and plug in your cities, dates and see what options come up and then read through the reviews and check out the hostel's own website or TripAdvisor for other reviews and photos.
If you aren't super fussy and just want a basic room, there are lots of hotels that have single rooms (albeit small ones) that won't break the bank - and if you are locked in with dates and willing to pre-pay, there are other chain hotels (like Ibis, CitizenM or Easy Hotel) that sometimes offer good deals online. The hotels are only an option if you are willing to eat on the cheap with takeaway from markets like Marks & Spencer - but if you are relying on having access to a kitchen so you can cook all your own meals, you'll have to take the hostel or Airbnb route. Airbnb has loads of places to choose from in all different price ranges - and if you don't require the full privacy of your own studio or 1BR apt, many of their listings are for an extra bedroom in someone's home. Couchsurfing is another option - sign up on their site and see if you can find a willing local host for your dates.