This is one of the most painful experiences in life. I am very sad for your suffering and can relate to the terrible anguish you are going through. He may have collapsed suddenly under a building nearby. Cats of his age are very vulnerable to this happening, usually because of acute organ failure - sometimes kidneys, sometimes pancreas or another vital system. Our cats are mammals as we are and sadly, suffer from the same illnesses. Because of his age, it is more likely that he suffered an acute medical incident than the other alternatives that are tormenting you.
Cats know when you love them. You loved him so much and he knew. With every day that passes it is less likely that he will return - though not impossible - and the not knowing is terrible. This may be the hardest thing you ever face in your life; Charlie would not want you to suffer, because you filled his life and heart with your love. They are extremely emotional animals and they know things like this. Make a little shrine for him perhaps, in your room, in his favourite place, with his favourite things, photos, talk to him there, and let your tears flow, because this so sad and so special and so big a loss.
When they sense that their time is up, they try to make it to safe dark hidden places that are familiar to them, where are they are safe from any attack from other animals near their territory. They just know innately to do this. They are far wiser and attached to us than many people realise.
Your grief will change in time. It won't ever completely go away. In time the pain recedes a bit, and then you are able to cope again, though you never forget, never stop caring. That's the price of a love like this, and you really really loved him - and he you. Honour the love you gave him and remember his joy that came from that. I wish you peace.