Merriam-Webster's Advanced Learner's Dictionary:
thirty
thir·ty /ˈɵɚti/ noun, pl -ties
1 [count] : the number 30
2 thirties [plural]
a : the numbers ranging from 30 to 39
• The temperature outside is in the high thirties.
b : a set of years ending in digits ranging from 30 to 39
• He is in his thirties.
• old photographs from the thirties [=from 1930–1939]
- thir·ti·eth /ˈɵɚtijəɵ/ noun, pl -tieths [count]
• the thirtieth of November
• seven thirtieths of a degree
- thirtieth adj
• her thirtieth birthday
- thirty adj
• thirty days/dollars
- thirty pronoun
• thirty of his classmates
- thir·ty·ish /ˈɵɚtijɪʃ/ adj
• He was tall, thirtyish [=about 30 years old], and had glasses and brown hair.
