This seems like a case of misaligned ideas and expectations
One of the most common pieces of advice is the concept of a Session Zero. In brief, the playing group sitting down together and talking through what expectations they all have and what ideas they want to try out.
It seems to me you have expectations that are misaligned with the ideas your DM is running with, a lack of a common vision that is typically associated with the lack of a Session Zero.
Your DM probably isn't doing anything strange
This is very much a matter of taste - sidequests are appreciated by some groups, and "finishing" side quests expeditiously is never a guarantee. It depends on the group, the DM, the setting and the campaign.
While the rules for levelling and encounters are quite clear, the DM can always dam the proverbial river of XP. Not every game needs progress as fast as a level every session or two, which is what seems to be most common.
From level 5, D&D characters are basically superhuman (if they weren't before), which necessarily shifts the scale and scope of the campaign. I have played more than one game where this progress has been considerably lengthened, to keep themes from the first couple of sessions relevant for longer.
When you mention "C rank" quests and teen character, I get clear manga/anime vibes. Within manga and anime, it is quite common for supposed sidequests to run for very long times, without the characters aging noticeably.
The specific system doesn't matter much, here; you are not playing a video game, but working collaboratively on a story. Devoting more than one "chapter" to something not apparently connected to the "main quest" is nothing strange, especially if the main storyteller (DM) has ideas to tie it into the main quest later on.
However, if an early time skip was promised
There is a disease among game masters, one I am definitely afflicted by, of getting enthusiastic about things actually happening in the game and leaving the overall plan by the wayside.
"I was only planning on doing this for half a session, but the players are interacting with the environment and NPCs so well, this is working wonderfully, let's keep this going for a while."
- me, many times
"Oooh, they made that assumption! Hmm, that's actually a good idea, could I fit that into the metaplot? Let's see...yes, yes I could. Let's run with that. I'll have to expand this arc a bit, but it's for the best."
- me, many times
Let us charitably call this "tunnel vision", getting too invested in details until the whole picture gets lost.
What's important to remember at these times is that roleplaying is a collaborative endeavor. The DM is the head arbiter and in control of everything except for the player characters, but he is human, and it is human to err.
Since you are collaborators, sometimes it becomes your responsibility to make your DM take a step back and consider the whole picture again.
Don't be confrontational
Now, it's important to remember that running a game is a creative endeavour. Why the Session Zero is so important is to make sure that all creative visions are aligned, and you aren't working at cross-purposes. This also implies that, as far as roleplaying goes, your DM is an artist and needs to be treated a bit gently.
Don't tell him he's wrong, because he isn't, and don't say there's anything directly wrong, but try to talk to him about how you would prefer more focus on the main quest.
Don't stage an intervention where you all gang up on him, instead try to treat him like a cat - don't approach too forcefully, employ a little reverse psychology, don't pet against the direction of his fur/vision, and try to offer a treat or two.
You can catch more flies with honey than with vinegar
- someone vastly more succinct than me
(Group dynamics are Hard, and psychology isn't much easier)
It's possible this particular group, DM or campaign are a bad fit
Different groups have different "styles". I've been playing with my primary group since the 80s, and the experience for a new player to join would be quite different than one without the shared bad habits of that group.
Different game masters have different "styles". I've played campaigns where I've swapped the role of game master with another player, each of us getting the chance to play and game master, and the storylines we've run have had our respective fingerprints all over them.
Different campaigns have different "styles". The same group and DM can, and usually do if they play together long enough, "switch it up". Change things to make the experience feel fresh again. This doesn't always make sense or feel fresh to a newcomer. Some campaigns are very different than others, even among printed material.
The most important part about roleplaying games is that they are, after all, games - you are supposed to be having fun. If you aren't having fun, something is wrong, and needs to be addressed. The worst thing you could have done, as far as I am concerned, is to quit the campaign and quit roleplaying, leaving the hobby with the idea that it's not for you firmly in your head.
Thank you for not giving up. As a first time player, coming here to ask this question is, in my opinion, going above and beyond. And, perhaps, a sign that the initial experience was good enough that you want to experience more.
There are other groups. If this group and DM aren't playing the game in a way you find fun and rewarding, and they don't want to adapt, check if there are other groups around. Try to play with more than one group, it will give you a better perspective of the challenges and possibilities of different styles of play.