We have raised an English Mastiff from a 25# 8 week old puppy and recently rescued a 6 year old 150 # former 'breeder'....there is not a lot of difference.  This is why I said to not be afraid to rescue a Mastiff.

Our first Mastiff was a male that we got as a puppy and throughly loved and enjoyed him for a short 9 years.....then we saw a female in the paper recently and went to 'see' her...within an hour she was sitting in the back of our suburban waiting to go home .  Absolutely love her....she is the same as our hand raised boy was even down to the way she sits on our feet and cocks her head.  ...