Check multiple hostel review sights. Others that are good, besides those mentioned above, are bugeurope.com and hostelworld.com. Some review websites include the age range of the person making the review. Read reviews by older travelers (30 to 60) to see what they have to say about the hostel atmosphere, and read the ones from younger travelers (18-24) to see if they keep mentioning stuff about what a great party scene it was at a particular hostel...then avoid that hostel.
YHA/Hostelling International is notorious for being the kind of place that older hostelers go to - they don't appeal to the young 'flashpacker' crowd.
Have you looked into other types of budget accommodation besides hostels? Convents and monasteries in Europe rent clean, barebones accommodation to tourists for really cheap. University housing during summer or holidays is fantastic - the University makes money by renting empty dorm rooms at a low cost to tourists (many of them offer private rooms with a twin or double bed, some even have private bathroom).
Even guest houses and B&Bs sometimes have a small single room, a room that doesn't have such a great view, or rooms that aren't booked and can be had cheaply at the last minute. Call them on the phone when you get to town and ask them to cut you a deal - never show up in person with your backpack or they'll try to charge you the regular rate.
Besides Rick Steves books on Europe, check accommodation listings in Rough Guide and Lonely Planet - both recommend budget places not typically found in RS guides.
I think you'll find a lot of your worries are unfounded anyways. I stayed in plenty of hostel dorm rooms with 18 year olds and found they often went to bed early because they don't have enough money to go out and experience the nightlife, so I wound up tip-toeing through the dorm at 2AM after drinking at the pub. That's the beauty of travelling in your mid-30s. You have a platinum card...they don't.