Do the beats become more erratic and less like a metronome with age?
It rarely is ever like a metronome even in our youth. The oscillations of a healthy heart are complex and constantly changing, which allow the cardiovascular system to rapidly adjust to sudden physical and psychological challenges to homeostasis.
How much does a healthy heart change with age?
This article does a good job summarizing the changes in one's heart as they age. I've summarized some ideas here, but I recommend you read through it.
Structural changes: There is significant structural change in the heart and vasculature; e.g. vascular stiffening, increased left ventricular wall thickness (within normal limits) and fibrosis with aging.
Functional changes: There are a number of functional changes and compensatory responses that the aged heart undergoes that diminish its ability to respond to increased workload and decrease its reserve capacity. Changes in maximal heart rate, end-systolic volume (ESV), end-diastolic volume (EDV), contractility, prolonged systolic contraction, prolonged diastolic relaxation, sympathetic signaling, etc.
Cardioprotection and Repair Processes: The cardiac mechanisms responsible for protection from injury and injury repair become increasingly defective with age, leading to accentuated adverse remodeling and increased dysfunction.