Maybe this has been said (I didn't read all the responses), but me, for example, being an Elder...am STILL a Deacon.
The age thing (unlike baptism), clearly isn't a hard rule, since we get 11 year old deacons now. But we are, undoubtedly, training young men. Ideally, yeah, 11-year-olds would be teaching... but...really? I'm a Teacher's quorum advisor...and some of those kids... like, you know... I don't want to use the word "retarded" or....anything...and not because it's politically incorrect to do so (which I shouldn't use it for that reason either)... but it's just a maturity thing. They're children still. And they act like it. The ages of 12 to 18 are HUGELY transitional years on maturing. We are training them. They get the priesthood young because they're old enough to start really learning and developing maturity and responsibility and all that...but they are NOT mature enough to just do it. So we train, and we train by assigning, and expecting, and giving responsibility to, and etc.
And this is seen through more than just passing the sacrament and so forth. Almost every Young Man at some point will end up in a presidency, unless they're just not willing or active. And the way classes run now...the Young Men teach. And, boy howdy, we struggle with that. But they're learning. They improve, sometimes, in some ways, bit by bit. They're learning and preparing for missions. They're developing the things they need to become what being a Deacon mean by being Deacons, Teachers, etc.
So, no, I don't agree they're treated as solely a sacrament passer...not in the least. But that is one way where they do learn, if nowhere else, responsibility, dignity, reverence, and so-forth.
They are also all assigned as ministers. Which...once again... teaching, watching over the church, etc., But as Young Men...they're in training.