Half your age plus seven
At twenty the rule allows 17 to 26 — nine years wide. At forty it allows 27 to 66, a span of thirty-nine. At sixty it runs from 37 to 106. The widening is the rule most defensible feature: it reflects the reasonable observation that a five-year gap matters far more at twenty than at sixty.
The rule gives a lower bound of half your age plus seven and an upper bound of your age minus seven, doubled. At 30 that is 22 to 46 — a range that widens steadily as you get older.
How the rule works
Its one genuinely interesting property is symmetry: the upper bound is the exact inverse of the lower, so the relation is mutual. If someone falls inside your range, you fall inside theirs. Very few folk rules survive that test, and it is probably why this one has lasted. It breaks entirely at young ages, where it allows ranges narrower than a year.
Questions
22 to 46.