Plutonium is much rarer in nature than uranium and is more commonly used for bombs rather than as reactor fuel. Like uranium, it comes in various isotope forms, with plutonium 239 the most commonly used. It is mixed with the much more unstable plutonium 240 to generate power.
It's my understanding that a horse's knees close around age 2- 2 1/2. My gelding JUST turned 3. A couple days ago in fact. Will a horse's knees almost certainly be closed by that age?
It also makes it easier for racing purposes. Thoroughbred races aren't open to colts (male horses under age 5) and fillies (females under age 5) until age 2, and some races, like the famed Kentucky Derby, are only open to 3-year-olds. But this presents some problems.