If your cramps are due to a Ca/Mg disturbance the cause may not be due to a dietary deficiency of the minerals themselves but a disruption of the regulatory system. This is highly plausible during the late winter/early spring as your body's vitamin D stores are at their lowest level of the year. Get a tan and eat some spinach and nuts.
However, I wouldn't jump to a complicated explanation so soon. Cramps are often just due to overexertion... maybe at 57 you aren't recovering as fast as you used to and now are overdoing it a tad. This is even more likely if you have recently increased your training.
You could also try elevating the legs a bit after a ride to improve lymph flow out of the lower extremities.