Question about Stevonnie

So if their age is just Steven + Connie's ages (like 12 + 13 making them 25 in their first appearance), what would happen if the two tried to fuse in their 50s or so? Would Stevonnie be in their 100s, or since Steven's physical age is based on how old he feels, would that affect what age Stevonnie would be, making them more or less the same age?