1. I've got a pair of Clark's Originals which I love. You'd be surprised at their suitability for summer; although they're higher-cut than most summer shoes, a suede pair will be nice and light. I also wear them throughout autumn and spring, and occasionally in winter with thick socks as long as it's not wet. They're really versatile!
2. Although suede boots will get quite dirty, I personally think it's part of their charm, but if you prefer a cleaner look leather may be a better option. It will also make them more suitable for winter but less so for summer, so keep that in mind.
3. I wouldn't worry about it, I'm only 18 and I've seen younger people wearing them