I also turned 30 recently. I think your issue is more of a mentality thing. You think your old so you start acting old. Lol. You have to change the mentality. You know the saying, age is just a number. You are as old as you feel. If you really think about it, 30 is young. There are plenty of people in there 40's and 50's that look amazing. As far as energy goes, try yoga. It is so amazing for your body and soul. It calms, energizes, strengthens, and stretches you. It has many health benefits. Hope the advice helps.
I agree with Mandy. It's not even physically possible that your health would deteriorate over night just because of two numbers changing.
There's got to be something else. Vitamin deficiency? Too large workload? Something in your personal life that stresses you out?
As for me I'm 35 and I've never felt better. I didn't start working out regularly until the age of 29.
I'm 31 and yeah sometimes I feel tired but that's only because of stress related issues. Otherwise I am energetic. I take vitamins. Eat healthy and drink lots of water every day ( except on weekends). I think maybe you should find some stress relievers. The more happier the mind and body the more energetic you will be.
Take a multivitamin regularly and coffee is a great stimulant pre workout. Make sure that you're taking in enough proteins for your bodyweight which is 1 to 1.5 gms per pound of bodyweight if you're active and up to 2-3 gms if you're trying to gain. Hope this helps :D salute for being a soldier as most of my family is its an honourable profession