Dr. Jones' Ultimate Feline Health Formula is designed with Adult and Senior cats in mind. It is safe to give to your older kitten - but since most kittens are very energetic and healthy, you won't see much of a difference in them if they do take the formula. So, it's generally not necessary - unless you are providing a home diet and would like to ensure they receive additional vitamins and minerals, or your kitten already has health issues.
In general, for long-term disease prevention, a good time to start giving Ultimate Feline is at 9 months to 1 year of age.