I am 7/seven years old.

For the most part, we use words for single-figure numbers, digits for anything above nine (ie eight, nine, 10, 11) - except with abbreviated units of measurement (eg: 3kg) and with percentages (eg: 4%). But never start a sentence with digits (eg: Fifty MPs have been expelled; Four per cent of the patients have died).

The same rule works for ordinal numbers: (eighth, ninth, 10th, 11th).

Millions and billions are spelled out, except where they are used with currencies or in headlines (five million people, 10 billion grains of sand, £5m). And remember that billion is widely accepted as meaning ‘one thousand million’ (not ‘one million million’). Fractions are written as words or, where appropriate, as a decimal (eg: three-quarters or 0.75).

Quote:

And remember that billion is widely accepted as meaning ‘one thousand million’ (not ‘one million million’).

I should have specified that this may only hold true in the US.