Someone started a great thread about SD neighborhoods, so this will give you a quick overview of each area in SD:
http://www.city-data.com/forum/san-d...reater-sd.html
I'd probably avoid Mission Beach, lots of USD students there. And I would only look into NORTH Pacific Beach near Turquoise St, the closer to Garnett Ave you are the more young and rowdier it gets. North PB or the Birdrock area of La Jolla would be good options for cafes, restaurants, bars, and shops and being close to the beach. Also check out downtown La Jolla, fairly expensive but it has a great downtown. OB is another good place to look, not quite the young, party crowd as PB but also very funky, hippy, and somewhat gritty; lots of tatoo and hippie types.
Downtown is also a great place for young professional in their 30's to live but it's nit close to the beach, if you can do w/o the beach then look into Little Italy or other areas in and around downtown.