Out of curiosity - why is it that children receive First Holy Communion at a young age 7/8 and are not confirmed until 8/9 years later and Adult RCIA has the adult wait for all the Rites at one time?
Good question.

It actually depends on the local Archdiocese.
Within my Archdiocese, children have First Communion normally at age seven, then Confirmation at around age thirteen. Confirmation is seen as a Sacrament that “completes” the Initiation Sacraments into the Church (being Baptism, Confirmation, and Eucharist) and that is to mark one as an adult in the Church and as a mature Christian.
However, technically Confirmation can be given to any that have achieved the age of reason, are properly instructed and are baptized. The original order of the Sacraments was: Baptism, Confirmation, and Eucharist and in the Eastern Rite, this is still the norm. Within the Latin rite, some Archdiocese have restored the Order of the Sacraments and this was done in past years. Our neighoring Archdiocese, for example, has made the decision that children are to receive the Sacraments of Confirmation and Eucharist at once. (as well as Reconciliation) in one year, at a minimum at age seven, but older if they need.
It is given to an adult in this manner as it follows the Rite of Christian Initiation for Adults, which is different than the catechism for children.
When a child is baptized older, it is at the discretion of the pastor/parish. For example, I was baptized at age thirteen. (long story). The priest decided that I should wait and be truly ready for the Sacrament of Confirmation, so I was Confirmed a year later after instuction with my peers, but had my First Communion at the next Mass after I was Baptized. My sister was seven when she was Baptized and made her First Communion with her peers. Within one parish, I have heard of the “Rite of Christian Initiation for Teens” but am not sure how that works or how widespread it is.
Hope that helped, and God Bless.
Jillian