Merrill has different life experiences than most characters in DA2, not less. I don't like when people say she acts like a child or "young." She was also easily in her mid to late 20s during the events of DA2.
I also know a lot of young people who are more "world weary" than I am at nearly 30.
I don't know, if you're looking for some coming-of-age romance narrative for the sake of being able to relate to it, Dragon Age hasn't really proven itself to be that kind of game series thus far. Alistair is the only one I can think of, and even then, not really?????
We then perceived those Characters differently. No need to get rude. I am not saying, what I say apllies to everyone of my Age or everyone with that kind of Perception. I just tried to explain what I meant in my Statement, which was rather ambiguously formulated.
Here my Explanation for why I (unlike you) perceived Merrill, as an Example, to have a Mentality I can really relate to and thus perceived her to be around my Age. You might have a different Opinion, but please remember that our Perspectives are different ones.
Merrill leaves her Clan behind in her first Appereance in DA2. For me that mirrors someone leaving their Parents and moving out from Home. She appears to be very lost in her new World, Kirkwall, and still relies on Guidance from others to really get along in this new Environment - these others are Hawke and Varric in her Case. She also left her Clan and especially her Keeper (who seems to take basically the Role of a Mother) on bad Terms, after a huge Argument, mirroring the Differences Teenagers have with their Parents. which can sometimes escalate the same Way. Over the Course of the Game, Merrill seems to mature slowly but due to the Problems with her Clan (Family, basically) she never quite arrives in her new Life and always seeks to prove her Worth to them - just like many Teenagers or young Adults, who have Differences with their Parents, seek to prove themselves. In the End, Merrill's Story sadly doesn't have a happy End, if we follow that Analogy, but she shows, that she has always loved and cared for her motherly Figure, Marethari, who reciprocates this Feeling. After that, though, she matures even more - Hawke's Stance towards her defines, if she gives up on her personal Dreams or keeps chasing them, just like young Adults come to a Point in Life, where they have to decide if they keep chasing their Dreams or go a safer Way.
I hope you can see, why I may have perceived Merrill's Arc as something of a "Coming-of-Age" Story I can easily relate to and can sympathize with. And, yes, while I do not restrict myself to such Romances, I liked Merrill's (Friendship) Romance best of all Romances BioWare has had so far. I But also liked Leliana's Path, which was completely different - but a Thing I liked about both of them (and Alistair as well) was their idealistic and somewhat naive Worldview and I hope, we will have at least one Love Interest amongst our Companions, who still has this Optimism.