I think when you understand how Jewish custom works you will understand there has been some misunderstanding as to exsactly what weaning meant back then. Not the way we understand it today. In Jewish custom there are two meanings for weaning.
The first meaning is the time a baby finishes drinking his mother’s breast milk. Today, that meaning is the most common understanding of weaning. But in Bible culture there was another meaning.
A child was considered weaned when he not only stopped drinking his mother’s milk, but after his mother taught him the milk of the Word of God. The mother would spend much time teaching her child the basics about God. Isn’t that beautiful? She would feed her child both physical food and spiritual food.
Weaning from the breast milk varied and it was generally around eight months to a year. Weaning from the milk of the Word lasted until around five or six years old. Scientists have discovered the incredible development of a child in ages 2 to 5. It is a very important time in a child’s life. No wonder they put forth the effort to instill the basic understanding of God in their children.
So many say around 6 years old.
•Manners and Customs of the Bible
•The New Manners and Customs of the Bible
Quote from Bible Customs.org ;
"How old was Samuel when he was weaned? To wean is to give up his mother's breast—child does this 8-9 months after birth.Do you leave the child in the temple to abide there after this? It takes about five years after weaning for the child to stand by himself. In the East, to wean is not just togive milk, buttogive the milkofthe Word. This is why the children sleep with the mother until she teaches them the milk of the Word. The mother gives the promise when she is married thatshe'll teach the child. They don't send her child to Sunday School. She teaches the children the Word, their religion. "Out of the mouth of babes and sucklings thou has perfected faith."When the child is able to praise God,then he's weaned. Some take 4 years, others 6. Samuel was somewhere around 5 to 7 years old. Samuel slept next to Eli in the temple. He was able to answer the questions. When God spoke to him, he thought it was Eli—he was old enough to know."
But we do find that some believe at the Jewish age of accountability is when he was brought to the temple as we find from Robert Seklemian of the Heroldmag.org ;
"When Hannah said she would give Samuel to the Lord when he was weaned, she did not mean weaned from the breast. Rather she meant when he was weaned from his mother, in the sense of being able to get along without her care. This would be when he was ten to twelve years of age. This is confirmed by the Jewish historian Josephus, who says Samuel was twelve when he was given to the Lord. It is interesting to remember that Jesus was twelve when he first presented himself at the Temple in Jerusalem. Also, it is the custom of the Jews to this day, to consider a 12-year-old boy as entering the age of responsibility."
Actually when I checked THE ANTIQUITIES OF THE JEWS By Flavius Josephus, chapter 10 paragraph 3&4 I found no mention of age when he was left, just when he started prophesying and that was 12years old. Although it could be presumed that this is when he was given over because it does state that when she brought him to be dedicated she rememberd her promise and gave him to Eli. So this would also be a safe assumption, compairing as well when Jesus was dedicated. Thus also reviwing tradition and customs.
Going back and looking at customs will give a more truer gage of when he was taken to Eli. Not at a young age of 3 as he would not of been able to properly be cared for and if he was dedicated as an infant the preisthood would of sent Samuel back to be raised for a time by his parents. But it is an understandable mistake when compairing to modern times instead of going back and putting propet interpretation into the context of study. It happens unfortunately all to often.
Hope this helps...