Looking at the picture, the horse has two permanent central incisors both top and bottom, a permanent lateral incisor quite well grown in on the bottom jaw, and one just in the process of growing in on the top jaw.
The two corner incisors, however, are still temporary (baby) teeth which will be next to be replaced by permanent incisors.
The horse is therefore 3 years old, rising 4 this spring/summer.
S
PS 8 year olds don't have baby teeth....
PPS That isn't an 'overshot' jaw either - most horses' teeth don't align exactly dead on...