If you are 40 years or older and desire the body of your youth it is possible to regain some of your form but it will require patience and some discipline.
You can gain muscle and lose fat over 40 & beyond but you cannot train like you did when you were in your 20's.
Just use some common sense before blasting the weights.
Factor 1: Your Training Approach.
As we get older our body can become susceptible to injury very easy so it is important to perform some warm up exercises before doing any lifting.
Use some cardio machines or lift an empty bar for 10 - 15 minutes to get your core temperature up and increase blood flow.
Factor Number 2: You Need To Have Perfect Posture.
Regardless of how little or how much weight you start with it is very important to perform each set with perfect posture.
Take your time with each set especially starting out.
It is not a sprint but rather a marathon.
You no longer have the flexible body of your youth so remember that, again use common sense.
Factor Number 3: Your Recovery Time Between Workouts.
Here is where the patience part comes into play; you will need to let your body recover more than you did when you were 20 years old.
Don't stress your body until it has fully recovered.
How long is that? Listen to your body...
the day after your work out are you really sore? If so how much? If the answer is a LOT then you need a longer recovery time.
When you are starting out go SLOW your muscles, your joints and tendons need to adjust to your workout routine.
All these factors are dependent on what condition your body is in, if you are overweight and have not exercised in years it is going to require a nice and easy slow approach..
..
but you can do it! A final note: the most important factor is to consult your doctor before starting ANY program, they will be thrilled you are exercising and will be able to give some pointers after considering your health.