I see many great ideas here and strongly agree Facebook is a great avenue.
Outside of all the "Online" answers I'd also suggest forming partnerships with other businesses in your local market that work with a similar audience. Do an open-house together, referral agreement or other simple partnership type of arrangements. I'd also suggest working with local realtors, PTA Moms and get active in the community events. These all helped me just as much locally as online advertising (if not more). Good Luck!
Your best bet would be to target those people on Facebook, and on Plenty Of Fish, these sites allow you to target their users on a demographics level.
Once you have created ads, make sure your landing page speaks to their pain points, and solves their problem. They will start paying attention to what you're saying, and after some time, you can sell them your services.
LinkedIn is the best bet as you can filter the exact target audience based on your locations and other parameters.
Once you have the list ready, you can approach via various modern marketing techniques.
Connect if you need detailed strategy.
I would highly recommend internet marketing as the most effective way to market your boutique hair salon. With internet marketing, you can target individuals within your demographic criteria and geographic location at an affordable cost. This can be done with an enhanced SEO strategy or content marketing. Write articles that describe your salon and market that through diverse platforms across the web, specifically in your geographic location with the "women above 40" slotted in (300-800 words).
I can provide many recommendations based on my experience with affordable services that exist and tips that can help benefit your salon. Many companies or individuals might recommend hiring a top marketing firm to help drive consumers to your salon, but I discourage this. You can easily do this yourself! Through my experiences working with small budgets, I have been effective in driving internet consumers to my sites from 0 users to upwards of 10,000 unique live users in just under 6 months. I would love to help you do the same!
Facebook is certainly a good option for this. But what kind of women above 40 are you looking to target? Are they stay-at-home moms? Professional women? Affluent? Savings-centric? This will make some difference in how you'll want to target both your advertising and your messaging.
I'm guessing that you are a local salon rather than a national network (but correct me if I'm wrong). In addition to Facebook, it's worth looking into your metro area's popular digital content -- newsletters, local blogs that are well-read among that demographic, etc.
I'm happy to take a call about this if you're interested in hearing more about how to create a blend of paid and earned media.
Which New Media Platforms Are Right for Your Brand in 2015?
You can create a Wordpress blog for business to create killer content and control your own advertising. Then you will gain valuable permalinks to articles to then create Facebook Ads from there.
If you have any more question feel free to reach out to me.
I am curious as to what specific problem you are solving for women over 40? What other characteristics do these women have? Your marketing tactics will depend on your strategy and a key step in developing the strategy is to have a clear audience profile(s). Knowing your audience will help you to understand the best place to reach them - it may be online, offline or a combination of methods. If your salon is local, partnering with complimentary businesses who serve your market is one way to reach your audience. This could be a joint offer, event or advertising.
It depends on what your goals are. There is no "one stop solution" for any type of marketing and, frankly, anyone who offers advice like that has no idea what they are speaking about and will hemorrhage money and profit out of your pocket.