That's a good point.
I'm very glad I waited to be in my thirties to seriously tackle long art projects (as an amateur, obviously).
I have way less time than ever in my life, but I know I can take the time I need, because it's unlikely my interests will change much in the next 10 years. I also feel I have so much more material to feed my imagination than when I was younger.
However, I don't regret the innumerable semi-serious, often unfinished, short or long projects I worked on before. Other than giving me an idea of what to expect with longer projects, they are also a good source of ideas that can be re-worked to fit in my current projects. I recently opened a box of manuscripts of novellas I wrote in 1996, style is terrible, but I'm going to cannibalize many ideas for an illustrated novel I'm starting to work on.
Theoretically, I think it's better to wait a bit (maybe not as much as me!) before lauching ourselves into a huge project; but I also know how frustrating it is to force ourselves to tackle smaller projects (possibly to the point of not being able to perform at all); so ultimately, in an amateur perspective, I would not discourage early large projects. I would simply say it's better to not have too high expectations, to not necessarily expect it to be a masterpiece, but maybe more a testimony of our progress.
(That's actually the stage I'm still at with comics, btw.)