The over-26 ones are still worth comparing, but it really depends on your trip then. (i.e. if you plan on using a bunch of not-highspeed trains, but want to be flexible and thus not book fixed low-price tickets early)

Yes, I did it 20 years ago in the pre-smartphone era and it was fantastic. The big advantage for that case was not having to plan, at all; you could simply wake up and decide where you want to go that day.

Amusingly when interrailing many years ago we managed to get on the wrong train at Ljubljana - we intended to go to Vienna but ended up on the overnight train to Belgrade.

The nice thing about an Interrail ticket being that even when you do make such a mistake your ticket is generally still valid!

It's not cheap if you are over 26. But if you want to travel a lot and not plan ahead it gives you peace of mind and most likely some moderate saving compared to buying tickets on the spot.