I estimate it to be 10 years.
... So I'd say Milo was at least 16 when the zombies came.
Milo was returning from deployment overseas when the the #### hit the fan, Milo would have had to be at least 18 (17 with parents consent) to join up then there is training (averages at 8 weeks for infantry) then actual deployment which can last for who knows how long.
Another thing to consider is Milo was trying to be funny when he was talking about buying a farm, the dude doesn't have the best sense of humor.
Another note; Soldiers cut tend to be near shaved heads and Jenny's art of Milo wandering the Wastland with his sisters pre-Halo has him with a shaggy head of hair on him. That and the fact that Milo had apparently warn out his shoes/boot (feet wrapped in bandages) also hint at a good amount of time.