Vickie, You will just have to decide for yourself, but the general rule of thumb is to take the date or age nearest to the event.Let's look at what you have.
You know from family tradition that Elvin and Alvin were twins and they are from Cross County AR.You have been told that the father's name is Doss Allen.The military record we found has a Doss C. Allen, born 7 May 1881 and he is from Cross AR.It states he has a wife named Dovie.
Cross County AR joins Jackson County Arkansas.
Jackson County AR has a marriage record of a D.C. Allen and Dovie Barnes 17 Aug 1902.They are both 21 and both from Remmel, AR. On 1900 Jackson County census there is a Dossie Allen, age 19 born May 1881 and also Dovie Barnes age 18, born Nov 1881.In August when they married he would have already had a birthday that year (May) and she would not have had (Nov).I don't think that there is any doubt that the marriage record is for the two people working as hirelings on the 1900 census in the same county as the marriage.
We now have Dossie, Doss C, and D.C. on official records.On two of those records it names a Dovie as wife.
In 1930, you have Dovie as a widowed mother of Alva and Elva.They are the same age, and have a bracket as if to indicate they are twins.
My opinion is that these are all the same people.The fact that her age is not correct on the 1930 census doesn't concern me.I can imagine a neighbor giving the census taker the info and saying, "Let me think. She hasa divorced daughter that is 22, and Dovie is a young widow, but I think she got married when she was about 14." Even the age or the names of the boys aren't right on the census based on the info given to you by relatives.Census takers took info from anyone they could, especially if the family refused to give it to them.Since this family doesn't appear to be on the 1910 or 1920 census, it is possible that they would not give info to the census taker at all.
There are several other things that you can check, but remember you are looking at records over a hundred years old and from a lot of different sources.Don't try to make everything match up perfectly.Some records are just better than others.