My daughter is turns 6 in Aug.  She currently takes Karate at her elementary.  I would like to send her for Aikido instruction.  Should I wait until she is older?

no reason not to as long as shes interested and likes doing this.

No worry on the age.

I'd opt for BJJ if it is around. Or Judo.

 
 

Look at the other juniors at the dojo. At this age being among her peers is important. My daughter was 12 when we started and was one of the younger students but since we practiced together she felt secure enough. The choice of martial art depends a lot on a child's personality too. She found Karate boring, too many repetitive drills. Judo, which was my first martial art, was a bit rough and tumble for her taste. Aikido was what she enjoyed and the defenses against being grabbed, tackled, etc. should stand her in good stead.

You generally don't want young kids to do Aikido since there is risk of damaging the growth plates in their bones with the type of joint locks we do.  Judo, karate, or BJJ are better choices until the late teens.  It is worth noting that the Ueshiba family sends their kids to do Judo until their teens.

If the dojo you are thnking about has a specific kids class designed around teaching kids without causing long term injury to them and they kids your daughter's age then go for it.

We started aikijujutsu with my oldest (boy) and middle (girl) at 4. We took this summer off but my son will most likely start back with BJJ, my daughter now 5 is undecided which discipline she wants back into.

Judoka first, then Aikido...

Judo you can start at any age especially with the rolling or falling aspect of it in the beginning. Kids think it's fun...