It is true that younger stock tends to taste better (Veal is a nice example) but if you go too young, investigation by law enforcement officials will become much more aggressive and citizen cooperation will increase drastically, (not to mention there may be unlawful retaliation and investigation that may be tacitly allowed by law enforcement) which will put a serious damper on business, so extreme caution regarding age is advised. Balancing the quality of ingredient with the difficulty of maintenance is something to keep a close eye on. It would be best to target ingredients that no other ingredient would miss, such as the poorly behaved, despicable, unlovable, or simply whose disappearance would not be noticed, such as the criminally entrenched or extremely disconnected (AKA "off the grid"). Generally ones that either none of them would miss or none of them would sympathize with the disappearance of. There are a pleasantly surprisingly large number of ingredients of this caliber which are actually quite young and would make good stock! Removing ingredients that other ingredients find threatening will also foster a sense of complacency among them, facilitating easier capture of future ingredients. (Disregarding the sense of unrest multiple disappearances will cause.) The cost of removing more dangerous ingredients is definitely a bit of a financial burden but the future ease of capture in an evironment where ingredients fear much less for their safety will allow the cutting of costs of more advanced abduction equipment and all without the cost of maintaining their needs as they sustain themselves in a natural environment where their only threat is you.
It's important to adapt to the sociological behaviors of the ingredients for your foodstuffs, but for long-term operation it is more efficient (especially if you have a much longer lifespan) to adapt their sociological behaviors to you. (And even if you don't, you will make managing the family business much easier on your descendants.)