What is the real reason so many drs discourage having a FL in your 40s? Does it compromise integrity of the tissues?

There is no age limit for facelift. A good candidate should have apparent laxity of the jowls and/or neck. This can occur at a wide range of ages. Now this isn't just some loosening of the skin. As you look at the jawline, you should see an irregularity in the line, and as you look at the neck, you should see drooping of the skin and/or muscle. If you have that, then you are a candidate for a lift. If not, then you should wait some. Be sure to see a facial specialist for consultation.

Thank you for your enquiry.
One of the reasons for advocating facelift later in life is that the difference between the preoperative appearance of the face and post-operative appearance of the face is more striking and therefore makes the intervention appear more worthwhile.
There is no scientific data to suggest that ageing is worse after facelift intervention irrespective of the age when the procedure takes place. If you are sensible about regular use of sunblock and protect your skin from the ageing effect of the sun, if your weight is generally stable and you are not a smoker, your ageing is likely to be slower and the impact on the facelift procedure will last longer.
I generally advise patients to examine their appearance in the near when they lie down completely flat. The changes in the characteristics of the facial features are relatively similar to the impact of the facelift procedure. If someone feels that this is the type of change they wish to achieve and this is the right time for them to pursue it, I believe that they have their own answer to the question.
Conversely if they feel that the degree of change between lying down and standing up is still relatively small, it suggests that the impact of the facelift procedure may be limited and they might feel that the acquired the associated scars without a degree of improvement that is meaningful to them.