As a slice of life/drama about young adults coping with various issues in their lives, this show is quite genuine, fun and well done. Every girl is unique and down to earth. They all experience highs and lows in their life, and most important - they grow.
Everything would have been awesome if not some issues that seriously interfered with my enjoyment of the show.
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This show feels too long. I've never imagined, i would say that about a series of 12 episodes, but Age of Youth has an extremely uneven tempo, where in the beginning it is excruciatingly slow and overextended. Adding the fact that the episodes are all over 1 hour long, it really tried my patience at times. And it's not like i didn't enjoy what was happening on screen but seeing some situations almost on repeat (like Kang Yi Na talking with older dude in the bar again and again, seeing Yoon Jin Myung sitting in that store without much happening and so on) didn't leave a very great impression to me.
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The show feels bloated with unnecessary dark and heavy drama which fails to feel genuine and realistic, because it's just too much of it stuffed in the later episodes. I like that all girls were treated almost equally, but i dislike the fact that 4 of 5 girls has some dark, heavy element or secret in their lives. It made it hard for me to suspend my disbelief. Some parts like the latter stuff with Jung Ye Eun's boyfriend, felt really poorly written and ridiculous, thus making supportive cast quite unimpressive compared to main five. That's why Song Ji Won felt like a superior character to me, because her parts weren't bloated with dark past or heavy drama. And i am not saying that characters shouldn't experience any hardships or can't have any past that haunts then, but when every character is like that it makes the show feel really unnatural.
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Other minor complaints:
First episode was too melodramatic. Creators didn't manage to introduce a new girl without a pool of tears.
The story about previous tenant was basically forgotten and never brought up again after the first episode, thus feeling like an afterthought.
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CONCLUSION
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Age of Youth is at its best, when it's just genuine slice of life show depicting simple, yet unique stories of 5 girls living together. Unfortunately it fails with its uneven pacing in the first half and lots of unnecessarily dark drama at the second half.

P.S. Extreme praise to Park Eun Bin for the depiction of Song Ji Won - extremely funny, well acted and lively character.