Maturity is more to do with the mind than with your age. Sure, your body physically matures. You cease to grow, you hit puberty. Mentally and physically women mature sooner than men, but mentally no one is mature when they turn 18.
"Not all people are supposed to be loyal to you. Some are meant to come along as a reminder to watch the company you keep."
If someone is rude or disrespectful towards you, politely excuse yourself and walk away. It sends a clear message to the other person that what they've said or done is unacceptable and that you won't tolerate it.
I knew I matured
I knew I matured - http://themindsjournal.com/i-knew-i-matured/
You should treat the janitor and the waitress with the same respect and dignity you expect for yourself and your loved ones.