I believe the Church has long considered the “age of reason” to be around age 7 (the time at which an individual can begin to distinguish right from wrong). Therefore, in most dioceses, young Catholics make their first confession and receive first communion at this time. Also, if a young person is baptized after this age, I believe they have to go through some personal preparation, rather than the baptism being a decision simply on the part of their parents. I’m not sure if there is a specific document that establishes this or not.
To get more at the the underlying issue of your student’s question, I would say that we should always being growing in our understanding of our faith and of the sacraments. But the fact that we will develop a greater appreciation and understanding later does not mean that we should not receive them now.