The Bible does not teach that there are different rules for salvation when one reaches the "age of reason" - whatever that is. Interesting theory but it is not biblical. No amount of works can save us. Nothing but the grace of Jesus Christ can save us. Works are important and should result from someone being a saved Christian however the works themselves do not save us.
I think the whole topic is academic. A saved Christian will benefit from grace and also have works. Cheap grace and ignoring works is not biblical either.