Is the bullet lead? A .25 ACP would be a metal jacketed round. To identify it, you really should clean it well, weigh it and measure it with a micrometer. It appears to be a fired pistol bullet, but it could be .22, .25 or .32. My guess would be a .32 S&W and the bullet should weigh 70-some grains. The age is impossible to determine. For instance, I load and shoot bullets cast from a mold made in 1877.