That is a very good point, but I would say my signature from 25 years ago looks nothing like my signature now, and my signature from 50 years ago looks like neither of the others. As people age, they tend to see changes in their handwriting and signatures. The only practical means of verification would be to make a comparison with several other specimens from contemporaneous documents...
As far as future signatures, most likely, for the foreseeable time, signatures will be electronic; we've all seen the little signature pads on all sorts of devices where either with a stylus or a finger, you sign for whatever the transaction is: a package, a purchase, verifying identity, etc. Sometime in the farther future, we will probably rely on biometrics: fingerprint or iris scans, voice or facial recognition, or some combination of these...
The Japanese have used a form of ink stamp called a Hanko since the time of Christ; it consists of a small block of wood or other solid material carved on one end with characters giving the name, and sometimes, the position, of the bearer; the Hanko is used by pressing the carved end onto an ink block and then pressed onto the document as a signature...
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