Numbers:
* For ages 1-9: You can spell them out (one, two, three, etc.) or use the numeral (1, 2, 3, etc.).
* For ages 10 and above: You generally use numerals (10, 11, 12, etc.).
Examples:
* "My son is three years old."
* "She's 15 years old."
* "He's a 25-year-old student."
Special Cases:
* Fractions of a year: If you are talking about someone's age in months or days, use numerals.
* Age ranges: Use hyphens when describing age ranges.
* Ordinal numbers: If you're writing about a person's age in a way that emphasizes their position in a sequence, use the ordinal number.
Examples:
* "My baby is three months old."
* "The children are between 5 and 10 years old."
* "The thirteen-year-old girl won the competition."
General Rule of Thumb:
* Informal writing: You can use numbers or spelled-out numbers more freely.
* Formal writing: It's generally better to use numerals for ages 10 and above.
Remember that there are no hard and fast rules. Use your best judgment to choose the most appropriate way to spell out the age based on the context.