43

This number is a prime.

The first time a twin's age in days, weeks, months and years can all be twin primes is when they (and their sibling) are 43 years old. Then, when they are 15887 days old, they are also 2269 weeks, or 521 months old. Note that some unfortunate twins were born in January, February, September or November. For these twins, after 15887 days they are 522 months old. They must wait until after their 1021st birthday before their ages in years, months, weeks and days are all twin primes. [Hartley]