How Do I Tell My Friend What I Finally Realized??

I have been friends with "Lola" for awhile now, we were inseperable last year and then late last summer she moved in with another friend, who Im not really friends with. She is now inseperable with her, and wont invite me to do things. So when she invites me and I already have plans she gets mad. On my 21st birthday outing last friday, I invited her and her roommate, her room mate is 21 but Lola is still oly 20 so she could only come with us to 2 bars. After hanging out with my friends 21 and up, I realized these are my true friends. they make plans with me in advance and keep them. As soon as Lola and her room mate came the mood of the group went down, and I realized that Lola is so much more immature than us. this is her first year of college, she worked for the last 2, while I have been in college for 3 years. So shes going to all the parties and has all the first year college stuff going on. I just dont know how to tell her that until she grows up and acts her age...