I saw this question and had to dig through my memory because honestly, 'The Last Song' is such a specific, time-capsule title that it really depends on which one you mean. The big one is obviously Nicholas Sparks's novel that got the Miley Cyrus movie. That book is a quintessential YA/New Adult crossover from the late 2000s/early 2010s. It's got the rebellious teen protagonist, Ronnie, sent to a beach town for the summer to reconnect with her estranged dad, the first real romance, family drama, and a tragic, emotionally manipulative finale. The themes of first love, parent-child reconciliation, and loss are pure Sparks formula, but packaged for a younger audience than, say, 'The Notebook'.
So, age-wise? It's solidly for 14 to 22-year-olds. The emotional beats hit hardest when you're closer to that experience of separating from your parents and figuring out who you are through a summer fling that feels like forever. The writing isn't overly complex, and the emotional payoffs are direct. That said, I'd argue it's less 'suited for' and more 'a product of' that specific age and publishing moment. Today's teens might find it a bit dated in its sensibility compared to current YA, which often has more edge or diversity. It works best as a nostalgic read for millennials or an intro to weepy contemporary for younger readers who aren't yet into darker or more speculative genres.
There's also a chance someone's referring to a different book with that title, maybe in fantasy or romance. But Sparks's version owns that space. It's less a genre and more a hallmark of a particular kind of sentimental, coming-of-age romance novel aimed squarely at the high school and college crowd.