I have my Dad's 521. He bought it new around 1950 or so.
I just found an old thread here discussing the production codes and it looks like Dad's was manufactured in 1948. He would have been 14 in late 1948 so that sounds about right for when he bought it.
As I understand it, it was originally introduced as the 521 T Junior Special as an introductory target rifle. It wasn't until 1958-59 that it was brought out in a somewhat simplified form as the 521-T and the Junior Special name was dropped.
No matter what, though, the 521s are fantastic rifles. The Lyman 57 sight that came standard was a great intro target sight and these rifles were accurate.
Dad's 521 was the rifle I learned to shoot on, and I LOVED shooting it. It seemed to natural to me.
Back in the 1980s I bought a bunch of different .22 ammo and benched it and found that the most accurate main line ammo, by a pretty wide margin, was PMC Zapper. I went back to the gun shop and bought 5 bricks of the stuff (same lot), which got me through the 1980s and into the 1990s.
One day my Mom came home with a yard sale find for me -- a Weaver C4 scope, new in box. With Dad's blessing I took the 521 to the local gun shop and had them mount a rail base on it to hold the scope.
Made a great rifle even greater. I used that combination to harvest a lot of squirrels in Pennsylvania.
I've not shot that rifle in years. I'm going to have to dig it out of the safe and take it to the range.