I found the following in POMS GN00204.035 Deemed Filing: 4. Deemed filing dateWhen applying deemed filing, the MOET for the deemed filer’s claim must be the first month he or she is eligible for the second benefit. When adjudicating the claim, MCS ma...

Another link that says spousal benefit is determined by age when spousal benefit is actually applied for. https://www.cityofvancouver.us/messenger/page/crunch-numbers-determine-when-start-social-security%E2%80%99s-spousal-benefit

This article states it is the age when the entitlement is available, I e. different ages used for reduction of own benefit and reduction in spousal benefit. https://www.socialsecurityintelligence.com/social-security-spousal-benefits/

This article implies the spousal benefit is reduced based on the original filing date, regardless of when the spousal benefit is available. https://www.aarp.org/retirement/social-security/questions-answers/switch-social-security-spousal.html

I totally understand the spouse's benefit on his/her own record is reduced when they file early.  My question is what age is used to compute the reduction in the spousal portion of the benefit when spouse A later adds the spousal benefit when spouse ...