Would I benefit from a facelift for the lax skin under my chin? (Photo)

Based on your photos, a necklift would be the only surgical procedure to undergo at this point. There are non surgical procedures that could help including Ultherapy, J-plasma, and RF microneedling. You may want to try those before undergoing surgery at this point. Be sure to see a facial specialist for consultation.

In your pictures, you do not appear to have any jowls, though the pictures make it a little challenging to see. For very subtle skin improvement, I would start with a fractional laser that is safe for neck skin like Halo, or an effective RF microneedling like Profound or a slightly more invasive Renuvion. While its not uncommon for patients in their 40's to require a facelift/ necklift, these pictures show that you have a nice jawline.
The other approach is almost prevention. You may find after a face/ neck lift that you will not age at the same rate as your peers and can help to ensure your privacy if you'd prefer no one knows you had work done.
I would think that a Face/ neck lift for you may be a little premature but it depends on your goals.