I have sort of the same question, but with a 7-year-old cat, and I've been thinking I should probably start feeding him senior food:
I have a 15-lb. 7-year-old cat, named George, who typically free-feeds on Hills Science Diet Adult Light/Hairball dry food during the day, and at night 1/2 can of regular canned adult Hills Science Diet.
I just adopted a 14-week-old kitten this weekend, named Ben, and I've been feeding him Hills' kitten dry food to eat during the day. And I've been giving him the other half of the canned food I've been feeding George (cutting the chunks up a bit smaller for his little mouth).
Both cats have hearty appetites, and George keeps eating Ben's dry kitten food, and I'm afraid of him gaining any more weight.
Reading your posts, I'm wondering if the best solution might be to give them both the same dry food that are good for all life stages, like semiferal mentioned...and then for dinner (the canned food), give George the same adult stuff he's been eating; and Ben a canned food made for kittens so I can make sure he's getting all the nutrients he needs. And I'll just try to get George to eat less and exercise him more (although he's already quite active).
Does that sound OK?