Wild animals eat 'wild' (or should I say live) food, I wouldn't call that cruel, it's just part of nature. All captive animals must be fed with the correct food otherwise they will suffer from health problems and even die.
Lizards such as Bearded dragons will live long, happy lives providing they are kept in suitable conditions, this basically means a proper sized vivarium and suitable temperature. Bearded dragons do eat quite a fair amount of vegetable matter. If you want something that doesn't eat insects then you might like to consider a Uromastyx, which primarily are herbivores. they do require a little more care however the housing requirements are similar to that of a Bearded dragon.
Finally, at what age you buy the animal is purely down to personal preference. My own preference is buy a young specimen and grow it on myself, young reptiles also tend to be cheaper.