I went to LA for my 18th bday just me and my 18yr old cousin! We had a ball! We were told to be careful of muscle beach area of Venice, and crompton. We walked lots of places around orange county although prob shouldn't have at night! We did Disney at day and at night (fireworks are fab!), universal, soak city water park and Knotts Berry Farm which is awesome!!!! Fastest wooden rollercoaster. Also went to beach , shopping in Santa Monica and Manns Chinese Theatre where we got our hands set in cement. Great trip that ill remember forever! Have a good time!
Things to watch out for - leery men at night, we walked fast and made no eye contact, and their drains at roadside are huge my cousin fell down one and badly cut her leg
Everyone will tell you to avoid Compton and Inglewood but then there isn't really any reason for a tourist to go to those areas anyway. DTLA south of the 10 isn't safe after dark, but again no real reason did you to be there anyway!