I'm nearly 60 and still pretty active but I do think that as we age we slow down and burn fewer calories just in the general moving about bit of life.
Plus, when kids are smaller we are SO busy that we never stop or sit down for long - always dashing to school/nursery then off to work, then mad dash home, running round after kids, not sitting down til 9pm. And we are concentrating on feeding kids healthy foods so we eat better without even thinking about it.
As we get older we slow down a bit, do less mad rushing around inside the house and out of it, we have more time to think about what we are eating so probably eat more (because we are thinking mmmmm I'm hungry), whereas before we'd be so distracted with small children mayhem that even if we were peckish, we'd not have time to fulfil that. So we probably ate fewer cals, and burned more, so stayed slimmer. We'd also be more toned as muscle mass is denser when young.
I think it's important not to try and reclaim bodies of our youth (or faces come to that) , and to embrace the ageing. If we are slightly more rounded in the middle, so be it. Embrace that our incredible bodies grew humans and are a bit stretched as a result.
For me it's all about health now. And not letting poor eating habits get out of control to avoid the over-fat middle that increases risks of T2 diabetes and other nasties.
So for me rounded belly =fine and gorgeous, fat belly=potentially poor health so needs controlling.
I really don't think there is a need for extreme dieting all the time, Just simply lots and lots of veggies, a good amount of protein (both to make you feel full and have a healthy gut) and low ish carbs (to avoid blood sugar spikes, bloating and largely empty calories). All complemented by regular exercise (weights a must for bone density) and Yoga (for balance), walking/cycling/swimming (for cardio).
The occasional fast day to kick start cell regeneration.