The old axiom “keep your left arm straight” was debunked long ago, most notably by the fine tour pro Calvin Peete. For senior golfers, trying to maintain a rigid left arm on the backswing can severely curtail your swing length and power.
A more limited shoulder turn restricts how far the left arm can swing back, while a loss of flexibility in the wrists prevents us from fully cocking the club into a strong, “loaded” position. Therefore, getting the club back far enough requires a little “fudging” with the left elbow.
Keep your left arm as straight as possible by stretching the left hand away from your body. If you can’t get the left arm any further back than parallel to the ground, go ahead and let the elbow bend a little. Don’t let it collapse to the point where the club drops down by your shoulders or around your neck, as this will actually cost you control and power.
Think of golf’s fundamentals as guidelines, not immutable laws. And feel free to break them, just a little, whenever necessary.