Mortgages die a natural death in fifty years (unless brought forward) under G.L.c. 260, §33. This statute, however, does not apply to registered land. See G.L.c. 260, §35.
If a mortgage states a maturity date and twenty years have passed since that date without the party or parties in possession during that period making any payments on the note the mortgage can be discharged in an action in the land court under G.L.c. 240, §15.