The info I gave you yesterday was from the IGI on CDs which I found in the library in Stirling. As you know, theoretically this is based on OPRs. The entries re Polmont were added by a member of the LDS so I went back to the library this morning and checked the OPR index which is on fiche. There is no record at all of baptisms of May/Mary (not Margaret) and Agnes in Polmont, but they are recorded for St Ninians.
To be on the safe side, I looked up the microfilm entries for all of them. They are all baptisms, no dates of birth are given. In each case it gives only the names of the two parents, the name of the child, the witnesses to the baptism, which in these entries notes the congregation of the church in which the baptism took place (in this case the Relief Church) and the parents' residence which in this case is specifically the village of St Ninians. It could have meant the parish of St Ninians but all the other baptisms have the name of the actual village.
I agree re the age gap but if Margaret was,say, 18 when she married in 1779 she'd have been 44 when Mary was born in 1805 - not impossible. It's a long gap between that and Samuel's birth in 1788 but she could have lost several children between Samuel and Agnes.
There are no other marriages of any AC/MS in the OPR index so it could be the same couple.
I don't have any details re their parents, but, again trawling through the microfiche index, an Alexander Christie in St Ninians was married to a Margaret Forrester and they had a child Alexander in 1756 and a Samuel in 1757 - same name pattern although of course not conclusive.
Hope this helps.