They typically fall out at 2.5, 3.5,and 4.5 years old. The two most inner teeth, top and bottom go first and then they move outwards from there. I have had them fall out as early as 2 and had them wait as long as just over 3 years to lose the first set. They should lose pretty much all of their teeth but rare to see them or find them. I have found quite a few since all but one horse I have owned has been a baby and have almost a complete set of teeth from different minis at the house. I just found them cleaning stalls or randomly they were left when eating in the food pan.
Yep, my gelding is 4 1/2 now and losing more teeth I noticed the other day. I've found about 5 of his teeth or molar caps in the manger. One time I thought one of the horses was injured because they had blood on their white, but it turned out they'd just been playing and Cody was losing teeth!