Well, that is rather subjective on many fronts. Most of our murals are painted by the parents, but kids can do them too. We suggest that kids over 8 years of age will probably do the best, maybe even up to 10 years old. It pretty much depends upon the child and his level of talent, patience, and hand/eye coordination.
I once asked my daughter-in-law what the talent level of our 4 year old twins was. She thought for a moment and said "Well, let's say they are in their abstract stage." Beautifully put. So I would say proceed with caution.
If you do let younger kids try to paint these or at least "help" paint these. I usually suggest the one of the parents paint the outlines and let the little ones paint inside the areas - that is probably safest.
BUT.... I had one customer call me to tell me about how her mural was coming along. She said her 5 year old daughter was just dying to help and kept asking her mom if she could paint over and over again. The mom finally gave in and let her daughter paint some areas. I asked the mom how it looked and she said "Well, I'll just send you a picture". She sent me a picture of the mural with the little girl standing in front of it. To be honest, I couldn't tell one way or another if the mural was perfect or not, but the picture of the little girl standing in front of her mural with the BIGGEST smile on her face said everything. She was SO darned proud of painting the mural - you know what... it really didn't matter if the mural was perfect. She was just beaming with pride!