I can't think of any reason why white pine scions wouldn't work, especially grafting onto black pines. The Japanese do it all the time, and you mention age, but I can't see any reason why that would be an issue either.
Since you're thinking of grafting, i would recommend looking up Ryan Neils demo on it. It's on youtube with all of his other lectures. I'll guess you've already seen it, but if you haven't it's great to watch. His video is grafting black pine to black pine, but it's still good to see and might help you down the line. And i can't remember for certain, so i'm sorry if i'm wrong here, but i think he went into grafting different species onto eachother, which would be perfect for you.