Every diocese is different. In the US the age can be anytime between 7 & 18, and the preparation is whatever the bishop says it is, he is the one who decides what “prepared and properly disposed” means. He usually delegates to the Catholic schools or Office of Catechesis to come up with the curriculum for sacramental prep, as well as for faith formation, RE, PSR, & Catholic school religion classes.
How the individual parish implements this–and they are not guidelines they are particular canon law of the diocese–is up to the pastor, and he usually delegates that responsibility to the school principal or DRE, who should be in communication with the Catholic schools office and the Office of Catechetics.
We are not talking about RCIA here, nor is this the Eastern Catholicism forum so let us not get off track.
Faith formation is not the same as sacramental prep. Some basic faith formation, 2 years seems to be a norm, should precede immediate preparation for sacraments. Sometimes these happen at the same time, even in the same class or group, but they are distinct.
That being said, I can tell you what the rule is in this diocese, you can go to cdob.org and click on synod legislation and scroll or search for the pages on the various sacraments if you are interested. It has the force of law and I can no more change it or claim responsibility or blame for policies that irk parents and candidates than I can for any other church teaching.
I also know that by and large most parishes follow about the same model: two years faith formation covering the Creed, Mass and Sacraments, Moral life in Christ, and Prayer, with the study of the relevant scripture and catechism topics related to those areas. Immediate preparation for Confirmation must include 3 parent-candidate-sponsor meetings, totalling no less than 3 hours, covering prescribed topics: the sacrament itself esp. related to Baptism and Christian initiation, the Holy Spirit, the Church, and discipleship. There is are required service component and retreat, to be decided by the pastor. Confirmation is in 10th grade or age 16 for baptized Catholic teens. In practice that includes a lot of children 10-14 each year who have been baptized in another Christian denomination, are prepared for full initiation with their peers, and celebrate first communion the same time as they make profession of faith and are confirmed. Plenty of the Catholic children also make their first communion in conjunction with confirmation, about 15% of the class.
Unbaptized children participate in RCIA as usual.
Faith formation should be offered in every parish in some type of formal class all through the school years. Our diocese specifically mandates a post-confirmation class and prescribes topics. Children receive sacraments of initiation, not of graduation, which means they are beginning their journey of faith and practice of the Christian life, and growth in knowledge of Christ, his Church, and his will for them.
I can also tell you that within the diocese there are dozens of parishes that do their own thing, add or subtract requirements, and interpret the prescriptions loosely, as pastors endeavor to best meet the needs of their own congregation.
You name a plan, and somebody somewhere is probably doing it, from once a month meetings, to twice a week, for one year or less, up to 3 years.