If you were born during or before 1960, you're a master. (Because your 40th birthday was on or before 12/31/2000, and therefore you were at least 40 on 1/1/2001, and thus by USACK Article 6, section C, 1.d., you're a master.)
If you were born during or before 1970, you're a senior. (Because your 30th birthday was on or before 12/31/2000, and therefore you were at least 30 on 1/1/2001, and thus by USACK Article 6, section C, 1.c., you're a senior.) (Most races don't have a separate class for seniors, but a few do.)
If you were born in any of the years 1971 to 1982, then you are not a master, senior, junior or cadet.
If you were born in 1986, 1985, 1984, or 1983, then you are a junior. (Because you will have reached your 15th birthday before or during 2001, and because you will reach your 18th birthday during 2001 or later, therefore by USACK Article 6, section C, 1.b., you're a junior.)
If you were born in 1987 or later, then you are a cadet. (Because you will not reach your 15th birthday until 2002, and therefore by USACK Article 6, section C, 1.a., you're a cadet.)