When booking a train ticket on the Austrian Railways website, the age options are "Adults" and "Children/Youths". What's the age cutoff for a youth fare? Most of the youth fares I've seen in Europe are for up to 26, but I want to make sure Austria is the same and I don't end up booking the wrong fare type.
I was about to ask why you didn't just check the website, but ticket fares are surprisingly well-hidden and there's no actual mention of age limits on the English site (update below). So, let's check Seat61, which has this to say about age limits on trains in Europe:
Children travel at a reduced fare... The age limit for children varies by country from under 12 to under 16 (and in one case, under 17), so see the list of child age limits below.
...although the child rate isn't always 50% off the adult fare.
It mentions several other things to watch out for, including the fact that adult fares are sometimes cheaper and that it can get tricky when you book international tickets. I highly suggest reading the entire page.
Anyway, the site goes on to list the child age limits for most countries, so without further ado:
Austria: Children under 6 free, 6-14 inclusive (= under 15) pay the child fare.
Since I couldn't find anything on the English site, I did some digging on the original ÖBB site and found the official definition:
Kinder sind Reisende zwischen 6 und 14 Jahre – ab dem 6. Geburtstag bis einen Tag vor dem 15. Geburtstag. Handbuch
Source: Handbuch für Reisen mit der ÖBB in Österreich, A.18.104.22.168 - Kind, gültig ab 01.01.2016
Loosely translated this confirms Seat61: child fares apply from a child's 6th birthday up to the day before its 15th birthday.
Note that a variety of family offers are available and international tickets also have cheaper rates for those below 26. Rates and reductions are listed in Preise und Preisberechnungen für Reisen mit den ÖBB in Österreich (PDF)
I don't know why you think it's generally 26 in Europe. In fact, with a normal ticket (Austria, train only, not international, no week/month card etc., no other discounts), the adult price is for age 15 and above. You can check this with Michael Borgwardt's link too, for any two stations within Austria. At least in Germany it's 15 too, so two countries out already. Italy is 11...
The 26 limit is important for some discount cards, mainly useful for people travelling much but irregularly (always different route/times), and in some cases for international tickets and/or combinations with buses. Eg. there is a "VorteilsCard <26" which can be bought for people younger than 26, ca. 30€ for a year, and means only the half price for all Austrian national train tickets within this year (and some discount for certain international tickets too). For people older than 26, this card is much more expensive. For Europe in general, there are the "Eurail" passes, where being younger than 26 helps too.
There are also discounts for families travelling together, etc., but this won't apply for one person.
Have you tried actually selecting the option? At least on https://ticketing.oebb.at/ as soon as you do, a line appears that has you enter the birth date and says < 26.
However, there are apparently some related services (e.g. http://www.postbus.at/de/Fahrkarten/Ermaessigungen_Verbuende/index.jsp ) that have a youth fare only up to the age of 21.