They can eat adult foood anytime after 4-5 months old. If they are spayed you should be feeding them a less calorie food now as the kitten food will put on weight quickly (especially after spaying).
Do a gradual change over - mix the kitten and adult together and over a few days put less and less of the kitten food till they are eating the adult food.
Most "adult" foods are actually labeled for "all life stages" and are fine for kittens. You will see a label on the can/bag that says something about the food meeting AAFCO standards for either "growth and reproduction", "growth and maintenance", "all life stages", or "adult maintenance". Anything with one of the first three phrases is just fine.
I've always fed adult food to my kits from the time they were about 2 months old, without paying attention to the labels Semiferal mentioned and they have always been very healthy and healthy. My 3-month old and my 4-year old eat the same Purina Chow and love it...
Although kittens as young as 2 months will eat adult food, I believe kitten food is better for them. It contains more calories, more vitamins and nutrients and is designed for growing bodies. Also, adult food may cause kittens to have diarrhoea. I believe kittens will survive on adult food but may not 'thrive' as such, with lots of muscle, the right amount of fat and a glossy coat.
Just to add, I think it also depends a lot on the brand of food. There are low-quality brands out there that kittens would definitely not thrive on but with better quality brands such as Nutro, Royal Canin etc, the adult food may be OK for all life stages.