I recently read a book where the main characters were barely described and you never got to know their ages. I tried to work it out, but the years didn't make sense. I found it very difficult to visualize the characters and I never became attached to them in the way I usually do when I read a book.
This distance between me and the characters also prevented me from getting genuinely involved in the plot, and even if I enjoyed the storyline, I didn't feel anything by the time I finished the last page.
Age shouldn't matter, but it's a fact I need to know to be able to recreate the scene in my mind and plunge into the story. I find knowing the characters' age makes them more credible, it explains their actions, vocabulary and train of thought too. I don't care if they're 20 or 50 years old, I just want to know their age.
This subject also makes me wonder why authors choose one age or another for their characters. Most books I've read lately have women in their late twenties or early thirties, but this may be because I love romances, and it seems it's the perfect age to find the love of your life, unless you're reading one of the classics where the perfect age will be early twenties or younger. Sleuths I've noticed tend to be a bit older, you probably need more years to gain the power of deduction and the expertise to identify a murderer by the way a leaf trailed in the wake of a long-distance lorry ended up on the victim's shoe.
What I'd like to know if this usually matches the writer's mental age. You know when someone asks you your age and you have to think twice? I always think 26 or 28 at first but then realize (shocked gasp) that is not my age any more, in fact it's been five years since. You would think I'd get the idea by now... But this is what I mean by our mental age, and I often wonder if this is the reason to choose one age or another for our characters, or if it's chosen to fit in with the plot.
Do you need to know the age of the characters? And why do you choose one age or another when writing?