Go natural if possible, else, be mindful of what you take and listen to your body (better if you can consult with a nutritionist, or YouTube at the least)
Day or night. I would say it's mostly about the discipline and routine to stay consistent. Iba iba kasi effect for different people. Me personally, I prefer evening after work.
Mid 40's here. I would suggest to start building on the discipline of exercise and healthy diet. Maybe start cutting down on vices para you don't get cold turkey by 40s. Hindi naman need to do all of these things pero it would help if you can ease into them a little. Syempre, kalakasan pa ang 30s so you'd want to enjoy things first.