Margaret McKellar (Brown) - Birth date for Margaret Brown/McKellar. Marriage of Margaret Brown /McKellar

I believe Margaret Brown McKellar is my 5xG Grandmother. She married Duncan McKellar in 1784 in Inverchaolain. I have the 1737 birth record for a Margaret Brown but I suspected that it may not be the same Margaret as the family described in this tree. The childrens birth records are available and it would mean that the youngest Duncan born 1804 would have been born when this Margaret was 67 which is unlikely. Does anyone have an alternative explanation?

My interest is in Duncan McKellar, Margaret's husband. I am trying to trace his father. I have a lead that Duncan may have had a brother Patrick McKellar married 1770 to Mgt Whyte and sister Janet McKellar married 1781 to a Charles Cleark both from Inverchaolain. The father's name might be Archibald but I have no firm proof yet.

I would be grateful for any help