This is a somewhat unfair comparison. The first picture is taken after a loss in a fight where he made more money than any MMA fighter will probably ever see in their lifetime, and did far better in that fight than any realist was predicting. He's smiling, his hair is freshly cut, his beard is freshly trimmed, and it's a well lit photo.
The second photo is after a fight in which he was virtually dominated from start to finish in a highly personal grudge match, losing his lightweight supremacy in the process. He has a very solemn expression on his face, his hair hasn't been cut in weeks at best, his beard is completely wild and unkempt, and it is a dark and poorly lit photo.
All that being said, the man has noticeably aged. This can be attributed to alcohol and substance abuse, which is obviously the case. The man was lit up like a Christmas tree for his public presser with Khabib for fucks sake. Not to mention the stress of all of his personal issues have had on him with the lawsuits and whatnot. Not making excuses for them of course, he brought them on himself.
Karma has it's way of following people around. I remember when a young, vibrant, and confident Conor told Aldo he looked like he had a stroke after his war with Mendes. Now Aldo looks younger than him. What goes around, comes around.