It varies by diocese, in the United States. In Tulsa, it was 11th grade, average age about 17. In Charlotte, it’s 9th grade, average age about 14. In some places it’s as early as 6th grade.
My personal preference would be for any time after about 12 years old, if the child went through the preparation process as decided by the local Bishop. I’m not a fan of age-cohorts or “seat-time” requirements. Just give them the exam or other assignments when they’re ready to do it.
Again, my apologies on the age for Confirmation. I’m not Catholic so I wasn’t quite sure even though I’m generally very familiar with RCC teaching.
Again, I feel the issue is giving the Body and Blood of our Lord to a non-member of the Church (Confirmation/Chrismation grants membership in the Church). Besides being Baptized Catholic and being 7 or older, what other requirement is there to receive? The RCC doesn’t rebaptize those from proper Trinitarian faiths, so should they receive if they are over 7? The RCC allows Eastern Catholics and Orthodox to receive in an RCC Mass, so someone from the East could bring up their infant or younger child (for purposes of this article 2½) and have them receive in front of everyone (it might look scandalous, but it would be proper).
I’m not 100% certain of this next item, but I believe it to be true. Your child at say 11 has been receiving in the RCC for several years, but you decide to visit an Eastern Catholic Church (ECC for short as there are more than just Byzantine). Would your child be allowed to receive as they have not yet been Chrismated? I suspect not, but an ECC child younger than 2 could receive in the RCC.
Obviously full membership was important (Confirmation/Chrismation) to the first millennium of the RCC as they offered Baptism, Confirmation and First Holy Communion in that order even to infants and this practice is carried by the Eastern Churches.