I think it makes sense to stay away from the lower end of the BMI scale as you get older, I would not increase the normal BMI range with - the SBMI is not widely used, peer reviewed, and based on less global data than BMI. It was a private project and I find it strange it uses unprofessional terminally such as "slightly chubby".
From the Times article
"I would prefer to see an older person overweight than underweight"
This does not mean an overweight BMI is preferable to a normal BMI, it is just more preferrable than underweight.
"Losing too much weight from your sixties onwards can backfire when it comes to your health, while a few extra pounds — and we are talking a couple of pounds here, not stones — might be protective."
This is also important - being the top end of the normal BMI scale will give those extra couple of pounds rather than stones, in most people.
Of course then the inaccuracy of the BMI tool needs to be accounted for, it is not perfect, but it is quite a wide range and should be used in conjunction with waist/height ratio, and how we look/feel generally about our physique.
I am older, I have gone from a morbidly obese BMI to 26. I have loose skin, I hold up my size 14 clothes and they feel like they belong to someone else, I am not used to being this small relative to where I started, I am wary of the weight I have lost and wonder if I should give my body a pause for it and for me to get used to it - it might be a sensible idea. But realistically, BMI aside, I know I am not yet a "healthy" weight and I am very far from a weight where it would be considered too low to be a health risk for my age.