I'll be 38 in a few weeks. My '08 WRX is stage one with nothing more than an AP and an SPT exhaust.
I bought my 2.5RS as my 30th birthday present to myself, before we had a WRX stateside. I modded the hell out of it. However, even when I was playing with Hondas back in my 20's I was always show, not go. I build my cars for power, handling and braking. I tend to like my cars sleeper. Nobody on the street has a clue that my RS can lap faster than a stock STi, and they don't need to know it.
However, It's really a personal choice. One of my business partners is 40, drives a bugeye wagon, and has a handful of cosmetic mods. He's got a v7 front grill and a rear wing. He's got rally armour mudflaps and body color side skirts. The car looks nice. It doesn't look rice. Something tell me that if you are even asking the question, you aren't going to rice out your car and your mods will be tasteful.
As for kids? We're starting a family in a year or so. At that point, we'll be returning the '08 WRX and getting a Forester XT. Now that car will get some real mods...