I once heard that when you start raising a child while the child is still under the age of 2, there is less concerns about Yichud later on. I have not found any source for this and was wondering if anyone ever heard of this and from where this is coming from.
I assume that this is based on the following:
כל המגדל יתום בתוך ביתו מעלה עליו הכתוב כאילו ילדו" (גמ' בסנהדרין דף י"ט ע"ב). משמע שדינו של המגדל (מאמץ) כדין יולד. השאלה היא האם זה "כאילו" ילדו, או "ממש" ילדו. ונפ"מ לאיסור ייחוד.
There are alot of sources confirming that and some even say that it counts for Pirya Veribya, and many sources explains that "Hamegadel Yusom" does not necessarily have to be someone that don't have a father.
But the "age" is not mentioned anywhere. I can imagine that if you take in a 18 year old boy it won't be considered "raising" him, so it has to be a starting point...
Any help would be much appreciated.