Help with use of 'had' in a sentence

[Kukai]

Hello. I was hoping someone could help please.

According to an ESL textbook I'm teaching from the word 'had' is optional in the second of the following sentences but I can't determine why this is.
I include the first sentence (about him being 89 years old etc.) as it may help explain why 'had' can be used {e.g. past perfect form perhaps?}

'He was 89 years old when he ran his first marathon.
He (had) started running at the age of 63 when most of his friends were thinking of retirement.'


Any insight appreciated on this grammar point. Thanks in advance!

[Alan]

The past simple is (especially in AmE) is frequently substituted for the past perfect where the sequence of events is self-evident.