Wolfgirl
Most vets know how to tell and if you ask them during a checkup can normally tell you about how old they will be when they are closed up entirely. With my horse, her growth plates in her knees were not closed all the way until she was three. Some horses are earlier, some later. I think the average age is around two, two and a half.

Natasha D
you will have to get an x-ray to know when. Most people think a horse is ready to ride by two, but they aren't. I don't like to start really riding and working a horse until they are about 3 or 4. I start ground work at 2 1/2, and from birth if they are born on my place. honestly a horse isn't ready to barrel race, work cows, do a lot of trail riding until they are around 3. I'd rather do that so I know they won't have joint problems and bone deformations later in life.

FQH5256
the spine and hock you will need your vet to xray, the knees you can tell when they are young and you feel the front of the knees you will feel a large space in between them. as they get older the space closes.usually between 4- 5 everything is closed up.

Greg B
As far as I know, X-rays or radiographs are the only way to know for sure.