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?
I think it really depends on the type of horse and its qaulitiy of life. Re: Retirement age - horses, not people! 15.2h horse about 28 I think (legs became too unstable to be ridden) - although mind was eager judging by his exuberant bucks! He lived to 36. 12h pony - no idea how old he was when we got him, only that he was old - but I would say he carried on being ridden until he was thirty.