It is often observed that the chances of a person suffering from measles in his or her lifetime are low if he or she has suffered from the disease in their early childhood. Justify the statement.

The correct answer is: Memory cells develop during measles in early childhood , subsequent encounters with the same pathogen elicits a highly intensified secondary / anamnestic response

This observation can be justified through the concepts of active immunity and immunological memory:
When a person is exposed to the measles virus for the first time during early childhood, their immune system initiates a primary immune response. During this initial encounter, the immune system identifies the pathogen, produces specific antibodies to fight it, and, most importantly, generates specialized long-lived cells called memory B cells and memory T cells.
These memory cells persist in the body for decades, retaining the "blueprint" of the measles virus. If the same individual encounters the measles virus again later in life, the immune system does not need to start from scratch. Instead, the pre-existing memory cells quickly recognize the pathogen and trigger a rapid, highly intensified, and specific secondary (or anamnestic) immune response.
This secondary response produces a massive quantity of antibodies so quickly that the virus is neutralized before it can cause significant infection or symptoms, thereby making the chances of suffering from the disease a second time extremely low.