What they do have is drive to get things done because they have not yet found the problems with their ideas.
To limit it to not seeing the problems is very very wrong. To limit it to any factual knowledge is very wrong. It is more about biology (different chemical balances during different ages) and how people generally relate to them selfs and life.
You of course have a point that a certain naivety also plays a part, but if you limit it to naivety, you reduce the inherent power of the youth for change to this naivety - like being young just means to be too stupid to realize that change is not worth the effort or something. And this does not only sound unreal, it also flies in the face of the myriads of things setting younger and older people apart except plain naivety.
The OP's suggestion aims to bring this inherent power for change to the legislation process. And that is IMO in principle an interesting idea. I think inno (no disrespect meant, but every time I have to go look how your name is written, I've gotten tired of it

) has voiced the biggest concern against this suggestion: That the young members of the legislative branch become mere pans of their older party members.
I want to expand on that a little:
- Legislation is about law. And law is complex and malicious because of its abstraction. If you have people in their 20s and you throw them into this hard business, they wil find them selfs probably crying for help.
- People in their 30s, who maybe have the abilities to be more independent, will have to buddy up with the older politicians to be able to continue their political career
- With a mere span of 20 years of available life time for the legislative and the early 20s unlikely to enter to begin with (how about studying something first?), there will be ah high fluctuation of personal in the legislative. Which further weakens its position against the other branches.
In conclusion, I think what is supposed to give the legislative power for change, will also make it exploitable for external powers (be it the executive branch or a political party). Some good spirit and motivation may be there nevertheless, but I fear it would largely go to waist if the legislative can not assert itself.
The judicial and administrative age groups that are proposed are rarely (major) politicians anyway because they have different skills. So the legislative politicians are likely to be unsuitable for the other branches.
Another problem I can see with forced retirement at 40 is likely to be an increase in corruption. Politicians will benefit far more than now in giving contracts to a particular business in the hope of employment when they retire.
Also good points.