I was under the impression that he decided to do that all on his own, as was one fashion at the time.
The idea was that baptism would erase all your previous sins... BUT a person might forget to receive penance/forgiveness/etc. for a sin or two after baptism, so it was advantageous to delay baptism until the person was so far gone that they wouldn't have a chance to sin between baptism and death.
Now Hypercaffeinated! Share and Enjoy!
Keep Saturn in Saturnalia!
You catch more flies with BS than you do with honey.