Such a statement is based on an inaccurate idea of what it means to be progressive.
If radicalism is a synonym for irresponsibility, then of course you grow out of it when you get older-
After pursuing random acts of violence, relationships with little commitment, (A la Abbie Hoffman and the Yippies) and having
the 'correct', 'most radical' politics- of course you become conservative if your politics are a shallow rebellion against
society in general.
It is interesting to note that environmentalism is exactly the opposite approach to society. Environmentalism critiques society
for having a lack of responsibility. It is little surprise then, that 80% (If I remember correctly) of Australian Seniors
call themselves 'environmentalists'.