Knowing a birthday and birth year are 2 different things. When young you will see the candles on the cake and know the exact age of your siblings. It won't be the result of calculations it will be from seeing the signs or candles that say happy 8th Birthday. When that stops you lose count of which birthday they are having. It just becomes happy birthday and only milestones are celebrated with a special sign or card. Adoptees are even less likely to see things that would ingrain a birth year in their mind. It is an added handicap in an issue with plenty of handicaps already.
I simply don't agree, a secure family is just that whether bio or not. Birthdays are landmarks, cards are sent, presents bought and families celebrate, don't they? I can understand JB as a young man getting his big sisters age mixed up but it has NOTHING to do with being adopted.