How reliable is the age on a census?

Question:

How reliable is the age on a census?

Need some help investiagting a possible discrepancy.
The 1901 Scottish census records my GG Gran Rachel Houston as 34 years old. But this would make her 16 when she gave birth to her son Samuel + he would be illegitimate. She subsequently would have married the father age 18 but her marriage ceritifcate gives her as full age which I'm told means 21. Is this correct for a female in Antrim in 1885?
I can't find birth or death records to prove anything so I'm completely stuck. Should I write off the census age as rubbish or is it possible?

Additional Details

3 months ago
Thank you everyone for the info it's much appreciated. I Guess it will be a mystery unless I find more documentation, but at least I know now that it is actually possible (which I'd kind of like to believe since I like drama lol.)

Best Answer - Chosen by Asker: 3 months ago
Thank you everyone for the info it's much appreciated. I Guess it will be a mystery unless I find more documentation, but at least I know now that it is actually possible (which I'd kind of like to believe since I like drama lol.)