understanding ages in 1841 census


I believe that the ages for adults were rounded down in the 1841 census. Is this correct? I am searching for the parents of my Allan/Alan Bowie who shows up in Campbell St Johnstone, Renfrewshire as 39 years old. If he was born in the latter part of the year 1801, December,  would he still show up as 39?
This parentage has been a stumbling block for years.
Thank you for any suggestions.
Janet Bowie Morgan

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