It's not easy to answer that question, because there isn't a single answer.
As my colleagues said earlier, far-right groups encompass a variety of sociodemographic profiles.
I've also done research on jihadist movements, and I can say most of their members are young men. Although there are some women, they tend to be in the background; they take care of logistics and communications. In other words, they have a supporting role.
When it comes to far-right groups, however, the profiles are a bit different. The groups are made up of older men and some women, who are taking on more visible roles although they account for a small proportion of group membership.
Oftentimes, upbringing is behind people's participation in these groups. Their parents may have had anti-establishment or racist views, and the children naturally tend towards those same views. Conversely, teenagers may try to move away from prevailing perceptions in society by joining fringe groups and sometimes skinhead groups. Little by little, they take on extremist beliefs that lead them towards violence. Others succumb to certain theories, within the confines of what we call echo chambers. That's where the responsibility of the web giants comes in. The technology behind social media makes it easy for beliefs to take hold, be reinforced and grow stronger.