I've had a TS since younger than 25. I've had a Secret since I was 17 (yes, 17). [I was a communications maintenance person -- we ALL had clearances and depending upon assignment the clearance was either S or TS and many of us were read into SCI]
There are a lot of people in the military who, by their AFSC/MOS/Rate, by itself, are going to hold TS or SCI clearances. There are a whole lot of people who, by their duty assignments, are going to hold TS clearances.
Do you think special operations people should be only 26+? How about crypto maintenance? How about nuclear weapons maintenance? How about aircraft maintenance on B-2's? How about missileers? and so on and so forth.
I hear you, most young military personnel have the maturity and sense of duty to understand the responsibility of having a TS. It’s the civilian types who think they’re so much smarter than everyone else (regardless of age) that do these things.
I had a TS/SCI for a decade. Moved on to other work, but don’t talk about even FOUO and Confidential stuff. Why bother?