I saw this definition of a true friendship on facebook and it actually struck some of my ADD cords. I can't take credit for this statement, but I think there's a lot of truth in this.
A true friend doesn't care when you're broke, what you weigh, what name brand you wear, if your house is a mess, where you live, about your past, or if your family is filled with crazies, they love you for who you are, a true friend can go long periods of time without speaking but never questions the friendship.
I know a lot of ADDers worry about friendships and I think this definition/description is a very healthy one for us with ADD/ADHD might want to consider.
1) true friends don't care about your financial status
2) they will love you no matter what you look like
3) they will not be offended if your house is a mess. I know this one from personal experience. My friends knew I was challenged in this area and they knew why. I didn't hide my messiness.
4) you can be completely open with your true friends, without fear of being judged
5) your true friends will encourage you, they build you up, cheer you on when you achieve your goals, no matter how small.
How to be a good friend when you have ADD/ADHD challenges:
1) love your friends unconditionally
2) forgive them for their mistakes and not always understanding you
3) make an investment into their lives
4) make sure to encourage them when they need it
5) listen to them and be open to their feedback
6) take ownership of your mistakes
7) acknowledge their feelings when you hurt them in someway
8) be generous with them
9) ask for clarification in conversations
10) if you have difficulty reading nonverbal cues, let them know
11) share your talents and gifts with them in a way that tells them you love them and mean something to you.
I don't think this is an exhaustive list of things that might help you develop good, lasting friendships. Once I got into college, I started making friends, friendships that have lasted for 12-13 years now. I strongly believe that these things helped me keep these friends. The important thing is to have friends, who accept you as you are and are forgiving when your foibles shows up. It is much easier to develop socially when you know that your friends will accept you no matter what. I attribute my having made friends since then to these friends I made when I was much younger.
I suggest that you look for people, who are inclusive and accepting of people from all walks of life. Also don't get caught up in age as a factor for friendships. I have lots of friends, who are older than me. I have friends, who are younger than me. And I believe I have friends around my age as well. Most of my friends have or had challenges themselves and have worked at overcoming those challenges.
Lastly, make sure you take care of yourself and be a good friend to yourself. Make sure you learn to love yourself and learn to forgive yourself. Never stop learning about yourself and your ADD/ADHD, continuously look for ways achieve your unique greatness.