Haven't traveled since I was 20 (now 30) are hostels still okay?
My first and last experience was from Vietnam Hanoi down to Saigon (four weeks) and from there I went to Bangkok, Pai, Chiang Mai, Koh Phangan, Koh Tao, ...
I met older people in hostels back then but really the crowd was about my age +-2 years.
I'd like to go to Indonesia and stay in Hostels but I also don't want to look like an old creep. Is 30 to old for Hostels and Partying?
It depends. Are you looking to travel on the cheap or creep on college girls and foreign countries with unknown police factors?
If it's the first one, don't worry about yourself. Don't let the thought of a random person maybe being uncomfortable with your just mere presence in a foreign country to prevent you from pursuing your dreams.
If it's the second, yeah you should probably stay home.