Learn English – Hyphenating spelled-out fractions with large numerators and/or denominators

A common rule is to write out numbers from one to nine (sometimes 10), and use numerals after that. This is also matter of style, and for example the Guardian style guide says:

Spell out from one to nine; numerals from 10 to 999,999; thereafter use m or bn for sums of money, quantities or inanimate objects in copy, eg £10m, 5bn tonnes of coal, 30m doses of vaccine; but million or billion for people or animals, eg 1 million people, 25 million rabbits, the world population is 7 billion, etc; spell trillion in full at first mention, then tn; in headlines use m, bn or tn

You should also use commas to separate the numerals into groups of three. So 1,002 rather than 1002.

But assuming you want to write any number in full, use as many ands as makes it clear, and use commas to list the separate groups of number.

• 102: one hundred and two
• 120: one hundred and twenty
• 1,002: one thousand and two
• 1,203: one thousand, two hundred and three
• 102,003: one hundred and two thousand and three
• 102,304: one hundred and two thousand, three hundred and four
• 1,000,002: one million and two
• 1,000,020: one million and twenty
• 1,000,200: one million, two hundred
• 1,002,000: one million, two thousand
• 1,002,003: one million, two thousand and three
• 1,023,045: one million, twenty-three thousand and forty-five
• 1,203,450: one million, two hundred and three thousand, four hundred and fifty
• 100,000,300 : one hundred million, three hundred
• 102,000,003 : one hundred and two million and three
• 102,304,567 : one hundred and two million, three hundred and four thousand, five hundred and sixty-seven

Although it is also common to say a instead of an initial one.

• 102: a hundred and two

And sometimes just hundreds are used instead of numbers:

• 2,502: twenty-five hundred and two

Years are different and have their own rules.

I have never actually seen either of those usages; what I normally see is either

thirty-one-month-old [child]

or, if less precision is needed,

two-and-a-half-year-old [child]

You could also say

[S/he] is two years and seven months old

or

[S/he] is thirty-one months old