Not actually legally adopted. However, our youngest son arrived (best Xmas pressie ever), on 24th December aged eleven and half months. We were foster parents, mainly for babies and small toddlers. This baby had been in children's home from birth as young social worker had hoped to get him back with his parents. This was not possible so he came to us for 'intermediate' fostering. Intermediate meant around a year.
He stayed, became the youngest member of our family, even using family name as he grew older and insisted on it. Adoption would have been very difficult for his biological Mum who did visit him (supervised visits only), also financially made little sense for us.
So he happily grew up in our boisterous, large family, we did occasionally have younger fosterings joining us. It was not until he graduated from Uni that he came entirely out of the local authority care system. As most of his life he had used our family name for his surname, there seemed little point in him deed polling a change of name then, All his certificates, bank accounts etc were in family name.
So, similar to an adoptive child, although not one. Yes, he always knew of his original mother, I insisted on that, the last time he saw her was just before he went to Uni, would only visit her provided I promised to stay with him.
To all intents and purposes, and emotionally he will always be my youngest!!!!