Is it more normal for senior cats to vomit occasionally than younger cats?

No, and this is one of the more common misreadings owners make. Occasional vomiting is not a normal part of aging in cats the way graying fur or slowing down is. It is genuinely more common in older cats because the conditions that cause it, like kidney disease, hyperthyroidism, inflammatory bowel disease and dental pain, are themselves more common in older cats. That is a reason to take it more seriously with an older cat, not less. A senior cat throwing up more than once a month deserves the same workup a younger cat with the same pattern would get.

When to call a vet: Any senior cat vomiting more than occasionally should get bloodwork, since the underlying cause is more likely to be a treatable chronic condition at this age.