Remember that if someone truly died at the age of 30 in 1900, they could have been born in 1869 or 1870 depending upon when their date of birth was in relationship to the date they died. If they were born in 4 March 1869, they would be 30 on any document in 1900 dated before 4 March and 31 on any document dated on 4 March or after.
So if a tombstone says the person died in 1900 at the age of 30, they could have been born in 1869 or 1870, if only the years are given on the stone.
Whether or not the age is correct in the first place is another matter.
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I recently found what seems to be an anomaly between a headstone and a census record. Her headstone reads that she died 15 Jan 1891, and yet I found her on a 1900 U. S. Census with her husband and some of her children. All of the other data on the 1900 record agrees with the 1870 census, 1880 census, her marriage record, the number of children, etc. The only strange thing about the 1900 record is that it states that the husband has been married 29 years, the wife 39 (this is the proper number). The 29 is fairly clear on the original record.