The primary purpose of a crate is as a housetraining tool... Depending on the amount of time you're able to spend with your pup, and how good you are at managing/providing for his toiletting needs, it's not actually necessary to use one at all. Most people find them helpful though (especially those who can't be home all day).
The length of time you'd need to use it is governed by how quickly your pup picks up housetraining, and how long the periods you leave him home alone are.
As for the 'no playing & running' prior to 18 months - IMO that is nonsense. On the contrary, that is how the pup
should be getting all of his exercise. It is forced exercise (i.e. on-leash walking or jogging) and strenuous stuff such as jumping or weaving (dog sports) that should be avoided until his skeletal growth is complete. The difference is in the overall stress level and continual pounding of sports/forced exercise versus the self managed breaks a dog takes when free to do so, as he is when running & playing offleash.
Most dogs of the size of a boxer would be pretty much finished with skeletal growth by 12-14 months, but many of this breed are slow-maturing and not actually finished growing until after 14 months of age. Hence, the 18 month guideline. Sounds to me as though someone, somewhere, picked up the 18 month bit, but got the rest of it backward
