Story: Pretty simple and beautiful, minus few minor subplots that goes nowhere/pointless (*docks points*). I really enjoyed the themes of parent/children relationships and marriages in this drama. Especially on re-evaluating in retrospect how the son (ML) became like his father (which was not what he wanted) and not regretting over the past decisions (of becoming young parents). Both ML and FL have good character development, realising what went wrong in their relationships and the process of reconciling. I also liked the second ML's story and kinda wished he got hitched with somebody (well, at least he has his 'daughter' :)). I think this drama was relatable in the sense that I am grateful, in retrospect, of my parents' relationships as a couple, which I thought was not perfect and not very romantic (*tmi tbh*), but in hindsight they were open and honest with each other (which what the main couple lacked). As mentioned, the themes of parent-child relationships kinda hit close to my heart, as such experience happens to me. (*very typical: my dad being nagging and both my parents and I not willing to be open with each other*)
Acting: Lee Do Hyun really carried this drama on his back, although some credits should go to Yoon Sang Hyun, for some of the more emotional scenes. He really sold the ahjussi trapped in a young person and the romance between his character and Kang Ha Neul's character (*SHE'S SO PRETTY*). (*imo their kissing scenes did feel a little uncomfortable to watch, as the main male character is a minor, although LDH himself is not*) I really think the LDH's casting was perfect, as they needed somebody old/mature enough who can pull off as a mentor-like figure to the students (e.g: his children) and of course young enough to make it believable that he is a high schooler.
Music: hello hello hello hello~... lol ok its pretty good for a romance drama and not that annoying despite that repeated 'hello'. It created melodramatic moments and raised the swooning romances.
Rewatch value: Honestly, my interest in this drama was piqued through a compilation of funny moments in the drama. I would probably rewatch those funny moments very soon. (*especially LDH being ahjussi and such a dad, IT IS ICONIC*)
Conclusion: *TLDR* 18 Again tells a story of a couple re-evaluating their relationship, the misunderstandings and miscommunications, such that this drama would not have started if the couple went for marriage counselling. It also *kinda* tells the story of how children and parents can misunderstand each other and the importance of parents being supportive of their children.