House training and doggie door

Hello
I will be getting a new puppy in the next 2 month or so and have a burning question.
Currently I have 2 older dogs that have constant access to the garden with a doggie door.
Should I start training the pup to use that door or just proceed with normal housetraining in the beginning by keeping him in an Xpen?
Has anyone started a pup on a dog door? Do you find that they are less able to 'hold it' once they get older?
Thanks for the help
Sabine