It was definitely a drama. Quite sad in parts, although it did have some touches of humour in it.
The plot is portrayed in the same manner as you have stated, in that order. I thought it was quite realistic, albeit hard to conceive of an "old baby". This was handled by him having some quite of aging disease. I forget the name of it, but I believe something like it actually exists.
In the movie, Benjamin reconciled with his father and became close.
As far as the play between young and old age, I guess there was some of that.
In the end, the clock finally gets dismantled.
I enjoyed the movie, but I haven't read the story, so I don't know whether to recommend the movie to you. Both my wife and I thought it was quite worthwhile and well-acted. The plot moved well, although it is a long movie. I think it was something like 2.45 hours.
If you do decide to see it, let me know how you liked it.