I was wondering if anyone knew how much food an ancient red dragon needs to eat each day, and ideally other age categories too.

I'm not sure if there's a definitive number, but based on their massive size I would guess it would have to eat a lot. Though whether it has to eat everything at once or has strategies for being able to space out its meals in smaller quantities would be an exercise for the DM. 

The red dragons are by their nature magical beasts. And when you accept that there is an intelligent magical flying lizard with the weight of a little building its food intake is entirely up to you. But.

Let us make some fairly rough estimates. The closest comparison to the red dragon on Earth is T-Rex. Its estimates are 4000 kg to 8000 kg in weight and calories intake 40 000 cal per day or more (assuming dinosaurs had the roughly same metabolism as today's mammals which elephant is the closest one). Now 1 kg of meat is roughly 1400 to 1500 calories depending on the type of meat and its fatness. So we can say T-rex needed to eat 26 kg of meat per day assuming he did not behave like a komodo dragon whose metabolism is slow.

Now to your question about the Ancient red dragon. T-rex is a large beast according to the Monster manual and the Ancient red dragon is gargantuan. So the dragon is anywhere from 4 times to 10 times bigger and heavier than the T-rex. And now the calories intake might differ a lot but for the sake of the argument let's say it increases 4 times at minimum and 10 times at maximum. So the ancient Red would have to eat 160 000 calories per day which equals roughly 110 kg of meat or one small orc or two small humans per day at minimum or he would have to eat 400 000 calories per day which equals roughly 285 kg of meat or 2 fat orcs or 3 fat humans at maximum.

So basically if you have a small town with 5000 population and a red dragon neighbour it will take roughly 5 years before the dragon eats you all. Yeah on that note we have to assume that red dragons are bad neighbours.

Very much one for a DM to decide on, preferably with some time to consider it.

I would say that, as a dragon is a very territorial animal, and red dragons doubly so, they will have adapted to need very large quantities of food, very rarely. A large crocodile or snake can live off 2 meals a year in the real world, and then remain largely inactive for the rest of it. A dragon slumbering on its gold hoard can probably venture out for a huge meal (such as a village) once every 10+ years or so and then settle back down. This adds to the anticipation of living near a dragon - you never know when it'll wake up hungry!

A dragon actively moving around will need much, much more food than that. I would expect an active dragon to need to eat a cow a day as the equivalent of rations, and would eat a small flock of sheep as a good meal. I feel like they would probably go for one big meal in the day rather than smaller ones, but that's just my thoughts and not based on anything!

After some Confused googeling on dragon weights and the average food to weight ratio of reptiles (which took godamm forever to find),I have found that they would require approx 216 lb of food every day,to not die.For proper nutrition they probally need 4-5 times that,which is a shit ton,it would take 7 humans daily just to keep them fed,and quite a bit of our weight is inediable making that number far larger.

Dragons are magical creatures and they explicitly can eat all manner of inorganic material, as well.  While chromatic dragons don't enjoy it the way that metallic dragons do, they're still capable of eating metals, precious gems, rock, and various other things.

stuffattackknightofrandom what was your figure for the weight of a dragon, as the 3.5 edition draconomicon says a colossal red dragon is 1280000 pounds.