While there is no absolute answer to that question, there are a number of reasons why talking about it "early and often" is the easiest and most beneficial way to go.
The world is a wonderful, awesome place to young children and they
are open and non-judgmental.
The longer you wait to tell the longer you have to get anxious about it (or worry that a well intentioned friend or family member will mention something before you do).
The sooner you start telling your child about how much they were wanted and loved (and the extraordinary lengths you went to have them the better.
The longer you wait the more your child will wonder why the information wasn't mentioned sooner: is there something wrong with coming into the world this way?
For more click on this link: Psychotherapist’s Innovative New Picture Books Help Donor, IVF & Surrogate Kids Understand “Where They Came From”.