This will be my only post in this thread, as I can already see the mocking of LDS beliefs in this thread, mostly by SteveVH, sigh.
Latter-day Saints believe that one must be called of God by prophecy to receive the priesthood. The Church of Jesus Christ has two priesthoods (or it can be said that the one priesthood has two divisions): the Priesthood of Aaron, and the Priesthood of Melchizedek. As has already been mentioned, the priesthood office in the Church of Jesus Christ that is more analogous to the Catholic office of priest or the Protestant pastor is that of “bishop”, who is the leader of the local congregation.
The functions of the office of “priest” in the restored Church have already been given, from our scriptures. They have the authority to teach the Gospel of Jesus Christ, to baptize, to administer the Sacrament (our equivalent to the Eucharist), to visit members, to ordain others to the Priesthood of Aaron, and to assist in missionary work.
We do not have a “professional clergy” in the Lord’s restored Church, meaning that our priesthood holders do not go through extensive graduate level training. Now, while I see nothing wrong with that sort of training, I also see nothing unbiblical about how Latter-day Saints view the matter. We believe that the Spirit guides those who are called of God to positions of leadership in the Church, and that God gives those who are called what they need to lead and to administer in His Kingdom (I am not setting this up as an either/or situation, fyi). We also believe that this is how the New Testament Church functioned as well.
As far as age and training, I find the words of God to Jeremiah (among other instances) to be helpful:
Jeremiah 1:6-9
** 6 “Alas, Sovereign LORD,” I said, “I do not know how to speak; I am too young.”
7 But the LORD said to me, “Do not say, ‘I am too young.’ You must go to everyone I send you to and say whatever I command you.
8 Do not be afraid of them, for I am with you and will rescue you,” declares the LORD.
9 Then the LORD reached out his hand and touched my mouth and said to me, “I have put my words in your mouth.
**
As far as sacrifice, Latter-day Saints do not believe that we are offering Jesus in our Sacrament, re-presenting His sacrifice, etc. Instead, as revealed in our scriptures, we believe that we are to offer to God a sacrifice of a broken heart and contrite spirit (which reminds me of Psalms 51:17). Our Sacrament is done in remembrance of Christ. Now, it is not “just” a symbol as many Protestants see their Lord’s Supper. Instead, while we deny a Real Presence doctrine, we believe that the Sacrament renews our baptismal covenants (as well as being a sacred covenant itself), as well as sending the Spirit to be with us.
Have a good day.