The rubber in all belts lose flexibility with age. When that occurs they will crack and stretch. On the 16 valve engines a broken belt probably won't bend the valves but a loose belt can break the nose off the drivers side cam. BTDT
Also, installing a new belt can damage the seal on an old, seemingly serviceable, waterpump when the retensioning takes place. Unless you like tearing down the front end more than once, I would recommend a new or rebuilt waterpump, front main seal and a rebuild of the tensioner with each belt replacement.