I found the christening dates for both of them on the FamilySearch site.
Hmmm. Never trust anything you find online, especially on Ancestry, MyHeritage, FamilySearch and similar web sites, unless it is an image of an original certificate.
I had a look at the marriage on FS. It is obviously an index reference to the original on SP.
However, it looks as if someone has looked for a possible birth and come up with a Thomas McCullock baptised in Barr in 1790, father David McCullock. However as you yourself have pointed out, there are a lot of Thomas McCullochs in Scotland, and there is no evidence on FS to prove that this Thomas McCullock is the Thomas McCulloch who married Elizabeth Gray. He could be, but he may not be. There are five deaths registered after 1855 of Thomas McCullochs who were born in 1790 plus or minus two years.
The same person has come up with a possible candidate for Elizabeth: Elisabeth Gray, parents Robert Gray and Janet McNinch, baptised in Newton in 1791. Again, there is no specific evidence that this is the Elizabeth Gray who married Thomas McCulloch. She could be, but she may not be. There are 29 deaths registered after 1855 of Elizabeth Grays who were born in 1791 plus or minus two years.
Further, the Thomas McCullock baptised in 1790 would have been just 18, and the Elizabeth Gray baptised in 1791 would have been only 17, in August 1808. A marriage of a 17-year-old and an 18-year-old was certainly possible, but it would have been very unusual. Most young men could not afford to keep a wife, never mind a family, until they were well into their 20s.
I note that the source of these baptisms is cited as 'Government record', which demonstrates a fundamental misunderstanding of the nature of the surviving pre-1855 records.