You are right about the Latin Quarter being too student-filled. It is also a little less "romantic". I find St. Germain des Pres the most quaint with its bookshops and large amount of cafes. It can be quite busy, too, but if you pick a side street to go back to, you can then pop right into the action when you feel like it. And it is a great location to walk to just about anywhere. In fact, we had planned to ride the metro a lot more than we did because by the time we figured out we passed a metro stop, we were already at our destination! St. Germain has all the charm you want. Le Marais is a little more hip right now I guess, but it has more saturated, too.

Hi
The 6th, 7th (left bank), or 4th might be good.
The 4th covers part of the Marais and the Ile St Louis, it is on the right bank, and it is absolutely charming.
It is one of the oldest areas of Paris.
It is also convenient walking distance to the Louvre, Notre Dame, and the metro will take you everywhere else.

We've stayed all over Paris, from Montmartre to Montparnasse, but on our most recent trip we stayed in the Latin Quarter. We're in our late 30s and we loved it.

The Latin Quarter is large and has diverse corners. I would avoid the area of the Boulevard St. Michel directly off the Seine--that tends to be a much younger crowd (a lot of teenagers on school tours). The crowd in the cafes surrounding Place de la Contrescarpe can be pretty young, too.

Based on my non-scientific observations, I'd say Montmartre had the youngest median age of the places we've stayed (probably mid 20s); Latin Quarter somewhere in the 30s; St. Germain always seems older to me, but the median age is probably late 40s--mid 50s. The rue Cler area of the 7th seems even a bit older--late 50s early 60s. And I couldn't characterize Montparnasse age-wise at all. It was probably the most diverse neighborhood of the bunch (age-wise).

Easy MJG . . . the Marais.

You're walking distance from the DJ bars of the Bastille (easy staggering distance back), while you can stroll over the bridges to Notre Dame, the Latin Quarter for dinner/clubs (then stroll back) - you'll love the area around Place des Vosges.

Good luck, have fun!