I read an article a while back...will have to dig it up.....that said that familes often saved up the children for a mass christening in order to save money. Evidently, the fee was the same to christen 3-4 children as for one.
I, too, got hung up on the triplet thing when I found three children christened on the same day but further research indicates that they were 7, 5, and one year old at the time of the christening.
I also have a brach of ancestors who are members of a non- conformist chapel in Wiltshire. They are all baptised as adults.
But, on average, I would say that most children were christened before their first birthday.