In general, older kids will be viewed as cooler kids, probably because they have more freedoms and do more complex/rewarding things. Because of this, the younger children will emulate them, for better or worse. If you think that these older kids are someone you want your children to look up to, then its good to have your children interact with them. If they behave poorly, keep your children away from them, for the same reasons. Older youth are more influential role models than same-age children.
Source: I see this effect as a leader in Boy Scouting constantly where it has good effects and bad effects. Bad effects tend to be more noticeable and quicker than good ones; you would be amazed how fast younger children will start using profanity if older kids are swearing. However, you can also be surprised at how an older youth can inspire maturity in younger children.
As far as it being better to spend time with same-aged children, I would not agree. Children need to get used to breaking and forming new friendships, because they simply aren't permanent. Having your child spent time with only children of the same age is artificial, and probably is not advisable.