Am I likely to be perceived as less successful if I start as an assistant professor at age 40?
I would say it depends. In the following question, JeffE resoundingly stated that age does not play into the decision whether to appoint somebody for an academic position. Yet, ETH Zurich (one of the world's premiere institutions in the technical sciences) even publicly states that they will rather not hire an assistant professor older than 35 (see the information box on the right).

Pragmatically, you may be a bit out of the norm. This may give you a small edge in some cases, and may be a small disadvantage in others. For every academic that is concerned about your age, there will be another one that values your industrial experience. I would say, if age is the only thing holding you back, then go and apply for assistant professor positions.


  Leaving a job in a research lab to start as an associate professor would in a way seem like a starting all over again.


Assistant professor isn't the same as associate professor. Both are not the same as "starting all over". In practice, in most places I have seen, professors on all levels are able to work quite independently, so none should be seen as an entry-level job. Assistant professor may be entry-level in terms of professorships, but it is actually a quite senior position in the wider context of academia.