What is the age limit for senior citizen railway ticket?

Can you please clarify for me the age limit that qualifies an individual as a senior citizen for railway ticket discounts? I'm curious to know if there's a specific number, such as 60 or 65, that's considered the benchmark for eligibility. Is the age limit standard across all railway services, or does it vary depending on the region or specific policies? It would be great if you could provide me with some details on this matter.