There is no age limit to learning anything new and language is no exception. Whether you are six, eighteen or forty, learning has no age bar and yes, you can definitely learn Russian.
The unknown terrain of a new language could be a factor that is stopping you from taking that step to learn Russian. Also, in your forties or a later age, you probably think that you are too busy, too old, or have other important things to do. However, if you are able to find a reason and good resources that can help you achieve the feat of learning Russian, you can definitely do so.