Whether senior golfers should let their left heel lift during their golf swing is really a question of flexibility.
The initial answer to this question is no, you should not let your left heel lift during your golf swing. Keeping your left heel on the floor, especially during your backswing, will create maximum torque, or twist, within your body and it is this torque that helps you generate power and therefore distance to your golf shots. Keeping your left heel on the floor restricts the amount of turn you are able to achieve in your hips during your backswing. If you are able to rotate your shoulders and upper body to 90 degrees to the right of their starting position (for right handed golfers) whilst keeping your left heel on the ground, then your hips will only rotate 45 degrees to the right of their starting position and you will produce a coil within your body, or torque, during your backswing. When you release this coil on your downswing, you will produce maximum club head speed and hit long golf shots.
However, if you are unable to effectively rotate your shoulders and upper body when you keep your left heel on the floor then this will cause you several problems.
Your golf swing requires a rotational movement in your body to be effective. If you are unable to rotate your upper body during your swing, you will tend to lift your arms up and pick the club up during your backswing, rather than making a turning action. This will reduce the power that you are able to produce, but furthermore, it will also make it difficult to swing the club head on the inside of the target line. The club head will remain much closer to the target line if you lift your arms up rather than turn your shoulders and as a result, you will make an 'over the top' action on your downswing where the club head moves over the top of your hands and travels to the outside of the target line.
As a result of this, you will now need to pull the club head towards the ball and across the target line as you strike the ball, making it difficult to hit straight golf shots. Turning your upper body on your backswing allows you to keep the club head on the inside of the target line and as a result, you are able to swing the club head back down the target line to the golf ball as you strike it. Keeping the club face aiming down the target line now means you will produce a straight shot.
If you are struggling to rotate your upper body well, then allowing your left heel to lift up during your backswing will allow your hips to turn more and therefore your upper body will turn more. However, if you are able to rotate your upper body well without lifting your left heel up then keep your left heel on the floor to create maximum power and hit your furthest golf shots.