Should Kids Play with Kids Older than Them?

My friend's daughter, who is 8, is spending a lot of time with girls who are 12 years old.  She's torn about how to handle the situation.

In one respect her little girl is getting a boost to her self esteem because the older girls keep complimenting her and teach her new cheer leading moves.  On the other hand she is neglecting friends her age and hurting her relationships with her peers. 

I had a similar situation with my son earlier this year when he was hanging out with a boy 5 years older.  It was much easier for me to say that they couldn't hang out because they didn't go to school together.  Its harder for my friend because her daughter is at the same school as her older friends and plays with them every chance she can.  By ignoring her peers on the playground in favor of older friends she is damaging her friendships with peers.  For example my daughter thought they weren't friends anymore because she (the 8 yr old) was always running off to find her older friends.

I can understand how cool it feels for a younger child when older kids pay attention to them.  However, its not good for younger children to always be with children years older than them.  Older children are going through things younger children can't comprehend, nor should they.  Can children form real friendships with children 4-5 years older than them?  As siblings possibly over time but as school mates, I'm not so sure. 

In Calgary the schools are divided more then the schools in BC.  In Calgary my children couldn't go to the same school because of the way they are divided: Grade K-4, 5-7, 8-10, and 11-12.  In BC they are in the same school: Grade K-7, 8-12.  I don't like the way younger kids are put onto the playground with older kids and I certainly don't like it when grade 8 girls are hanging around grade 12 boys. 

In my girlfriend's case I can see how this friendship gives her little girl confidence but I believe it should be monitored and limited so that better friendships with peers have a chance to grow.