Confirmation is a Sacrament that can be received at any age, and different dioceses have different practices. It grew out of the Sacrament of Baptism, and indeed is often conferred with it, but it does have added significance, which its separate celebration brings out.
It takes its inspiration from the coming of the Holy Spirit on Our Lady and the Apostles at Pentecost (Acts 2:1ff). You may remember that after the ascension of Our Lord, they returned to the upper room (presumably the same room where they had celebrated the Last Supper) and there they spent the time in prayer. On the Jewish feast of Pentecost, they experienced the coming of the Holy Spirit in tongues of fire, which rested on their heads. The results were dramatic. Whereas before they had been, well, in retreat, I suppose we might say, afterwards they were ‘filled with fire’ and immediately went out to tell others about Our Lord – to ‘evangelise’ with dramatic results.
Confirmation is administered by a bishop or a priest, and by its power we are filled with an increase of the grace of Baptism, but moreover, a grace that enables us to be witnesses, ambassadors, for Christ. You might say that we are commissioned as representatives of the Church, and this naturally carries with it the responsibility of keeping ourselves informed about the scriptures and the teachings of the Church, as well as maintaining our life of prayer.