Term lengths are usually chosen in round numbers and then anchored to the buyer's age. The obligations a policy exists to cover run on their own schedule, and that schedule is the better guide.
The policy exists to cover a shortfall
A death benefit replaces something specific: a mortgage that still has to be paid, income a household depends on, or education costs already committed to.
Each of those has an end date that can be identified today. A mortgage amortises to zero on a known schedule, and a child finishes education in a known year.
Cover is needed until the last of those dates, and not usefully for long beyond it.
Age enters the calculation only through its effect on price, since the insurer charges more for the same period as the applicant gets older.
Round numbers rarely match real dates
Twenty and thirty year terms exist because they are easy to price and easy to sell, not because household obligations end on those anniversaries.
A mortgage taken out three years ago and a policy bought today are already three years out of step with each other.
Buying to the nearest round number either leaves a gap at the end or pays for years of cover after the obligation has gone.
Ending too early is the expensive mistake
If the term ends while a need remains, replacing the cover means underwriting again at an older age and in whatever health the insured is in by then.
That is precisely when replacement costs the most, and for some applicants it is no longer available at any standard rate.
The extra cost of a slightly longer term at purchase is small next to the cost of requalifying a decade later.
Ending too late is a smaller but real cost
Cover held past the point of need is premium spent on a benefit nobody is relying on any more.
Term policies can be cancelled at any time without penalty, so the excess is recoverable by simply stopping payment, which makes over-buying the safer error of the two.
The asymmetry between the two mistakes is the reason careful planning tends to err toward the longer term.
A policy that is cancelled early has still done its job, because the household was protected for every year it was actually exposed.
Several obligations mean several end dates
A household rarely has one horizon. The mortgage, the years until the youngest child is independent, and the years until retirement savings suffice all end at different points.
Writing those dates down converts a vague question about term length into an arithmetic one with a definite answer.
Where the dates are far apart, separate policies of different lengths fit the shape better than a single compromise term does.