The niche sounds completely viable. You really only need to worry about search terms if your only plan for traffic is organic.
You can put together a site with few search terms and still have it do well if you use other traffic methods to get visitors, and in this day and age there aren't many left who rely 100% of SEO.

If you are going into weight loss is certainly a good idea to go into a sub niche. Or even a sub sub niche!

A lot of the women over 40 searching for weight loss help will not use the fact that they 40 or more – or even the fact that they are women – in their search terms.

So someone searching for "weight loss fruit diet" could well be a woman over 40 as the search term does not indicate this one way or the other.

But targeting your site at this demographic will inform the design and content of your site.

And don't forget that lots of your traffic could come from social networks – if you work them the right way. And also from forums which you post in regularly.

So don't just think of Google – we all know what a big mistake that is these days.

Malc

I agree with Mark & Malc. Take their suggestions serious.

In addition to the duo suggestions I will suggest you search for sites that are service your audience already. I bet you they are out there.

You can then do intelligence research to know how they are serving the people in the market.

You can do intelligence analysis to know how they are getting people to their website.

For organic search traffic you can use semrush.com to see keyphrases they are ranking for. Definitely it won't be weight loss for women over 40. Pay attention to their phrases.

Same thing for paid traffic. Semrush can give you some info to start with.

My 1 cent.

Lateef

Not every niche will have people actively searching for the exact term.

Most people aren't going to be so specific in their search that they type in "weight loss for women over 40" BUT I'm 110% certain that there are women over 40 DO want to lose weight.

Approximately 1 billion people worldwide are overweight.

68% of American adults are overweight.

The market is estimated to be worth over $120 billion.

85% of dieters are female and they make 4-5 diet attempts every year.

Sources:
Weight Loss Statistics - Worldometers
WIN - Statistics
Obesity Facts, Statistics, Causes, Symptoms, Treatment - MedicineNet
100 Million Dieters, $20 Billion: Weight-Loss Industry by the Numbers - ABC News

Niche research isn't just about keyword volumes. Some things just aren't searched much on Google despite being popular / in demand.

It's about examining examining the wider market, looking at data, spending and insights, checking what products are on sale, evaluating the competition, checking on who the current competition is in the niche, making sure your target audience is easily reached and more.

The good thing about targeting a specific demographic is you can literally advertise anywhere that demographic can be found online even if the site is not about your niche topic.

Think about TV and radio adverts. The advertisers target the demographic of who watches / listens to the show. You can do the same online. You're not limited to advertising on weight loss sites but anyone where women over 40 are found.

When looking for products don't think you need to find products which explicitly state they are for women over 40, a weight loss product / supplement helps people lose weight regardless of sex or age.

Make sure to find out what the real concerns of this demographic are. Sure their main problem is they want to lose weight but WHY do they want to lose weight?

Is it because their husband is loosing interest in them? They don't get attention from the opposite sex any more? They can't find into those skinny fit jeans any more? They are getting worried about their health?

Seek them out on forums, blogs, social media etc and see what their concerns are.

What questions do they ask? These are problems you should solve through your content / products.
What language do they use? You should use it too in your content.
What causes them to want to lose weight? Use these pains in your sales copy and content.

You're on the right track being super specific and targeting a particular person / demographic rather than trying to be general and market to everyone.

The more people you try to appeal to the less people you appeal to.