That´s difficult, I know the ordeal from a time when I modded a roleplay avatar to be his father, I don´t have mature skin recommendations, but maybe if you can´t turn anything up, the route I finally went might work for you - I pushed the shape sliders of my 'normal avi' so long until I was satisfied with the look and just kept the same skin adding 'wrinkle tattoos' I found on MP, I even found a grey beard that looked good enough, and was quite happy with the result, he didn´t look like a model at all but exactly as I had imagined him. I don´t think there are many results for grey beards or wrinkles, so searching MP for it shouldn´t be too hard.