Are the sentences below correct and natural? Thank you.
1. He has not had a physical checkup since his 60th birthday. Not having physical checkups regularly may cause serious problems for senior drivers.
2. He has not had a physical checkup since his 60th birthday. Senior drivers without having physical checkups regularly may cause serious problems.
(1) is fine; (2) is not. It can be rephrased as follows, as an instance of deliberately delicate wording:
Seniors who drive without having regular physical checkups may cause serious problems.
I won't try to explain why "without having checkups" should sound more natural as an adverbial phrase rather than an adjectival one.