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?
The average age a horse fully matures, both mentally and physically, is 4-5 years. Although you can start a horse under saddle before 4 or 5 years of age, it's important to remember they are still developing at that age and therefore your training regimen must not be too severe for their developing bodies to handle.