I am 37 and have had long hair my whole life. I've grown my hair to my knees twice after having cut it to where my bra line is on my back. I have found that Hair, Skin, and Nail Vitamins really help your hair grow faster and stronger. Rubbing a raw yellow onion on your scalp once or twice a week and letting it sit for about 15 minutes before washing it out also makes your hair grow faster and has known to stimulate growth on the scalp where their hasn't been any for years. Another helpful solution I use is a mixture of 1 cup vinegar and 6 cups water. I rinse my hair and scalp with this once or twice a week, especially after rubbing onion on my head, and I noticed that after each rinse my hair is softer and more managable.
My grandfather lost his hair after battling cancer. His remedie that made his hair grow back even thicker than ever before was a mixure of witchhazel and iodine which he massaged into his scalp a couple times a week.
As for beauty products that promise to increase your hairs growth and strengthen your hair, I have found nothing that works as well as those that I have mentioned above and the cost is next to nothing compared to the price you are going to pay for products with a whole lot of chemicals.
Here is another great tip. If your hair is dry and brittle you can take olive oil and heat it a bit and then rub it on your hair (not your scalp) and leave it on for an hour or so once a week and you'll find that your hair will eventually become hydrated once again and won't break off as much when combing or brushing through it.
I hope you try one or more of my remedies above before purchasing chemical ladden products. They have always worked for me and I have never had an allergic reation to any of them, and the cost is pennies compared to over the counter solutions.