> For example, a person who is 30 years old has entered their 30's. This is a different linguistic construct. We have decades like the 1990's, which are like t

> For example, a person who is 30 years old has entered their 30's. This is a different linguistic construct. We have decades like the 1990's, which are like the 30's. Linguistically, we have: 1800's = 1800-1899 19th century = 1801-1900 (since AD started in 1, so the first 100 years, or century were 1-100) One of those forms is linguistically similar to your 30's, the other is not.