From an interview with Lufthansa magazine:
It used to be said that the voice reaches its peak at 50, but much has changed. Life expectancy has increased, people feel younger. Plácido Domingo is (nearly) 80 years old. So it does not look like this magical line still exists. I do not want to take any restraint and prepare myself for the old bit. But I do not want to cram my schedule so much that I can not enjoy life anymore.