Personally I don't think that they clock us any easier than anyone else. They are not born with any special perceptive qualities that anyone else. Generally adults are often too polite to make any comments towards a TS person who they have clocked, and if they are with someone else they more often than not will simply say something under their breath to the person they are with.
Teens on the other hand have different values, values they are developing, teens need to interact with their peers, they need to be apart of the group they identify with and be accepted. Consequently, these busy lives usually don't have the time to analyze those who don't fit into their idea of what is acceptable, allowable, cool for the group they associate with.
A number of years ago I had an encounter on a school bus where a lot of the students know me and about me. On this occasion three new students happened to be going home on the bus I had to drive as the driver had called in sick at the last minute, usually no big deal. Now one of the rules that I teach all new bus drivers is that they will be privy to many conversations between students, and they will here some things that they may find questionable, funny, or even somewhat rude, therefore they should ensure that while they must listen they shouldn't "Hear" everything.
The three new students sat at the back of the bus (The Cool Section, the place where the coolest kids get to sit). from their conversation it was obvious that one of the new students had clocked me as we're in pretty close quarters and immediately started to throw insults my way. I didn't hear everything, but I did catch the odd "He must be a ->-bleeped-<-", "What a pervert", and even a "Hey 'It' !". Not nice stuff, but I ignored it as I overheard other students rebuking them, and they eventually stopped. Now had one of these new students gotten on the bus by themselves, then I doubt that they would have said anything but being with a peer group, and needing to fit in, it was almost required that they say something.
Just my thoughts.
Steph