Logic/Proof:
Yoshi's Island:
-Mario and Luigi = twins = same age
-Bowser = able to talk, but "baby talk"--sentence fragments and "cutesy" sounding = toddler = older than the Mario Bros., but only by a few years. Unless, of course, Koopas age at a different pace; but relative to their ages then and now, it seems pretty close.
-Yoshi = very, very young looking, somewhere (I believe) mentioned was a baby = older and more mature looking than a "baby-baby" Yoshi (See SMW) = toddler age = around same age as Bowser
-Peach = not mentioned at all = most likely not born yet
Super Smash Bros. Melee (Yes, there, ARE some inconsistancies, but other than the info PROVEN to be incorrect, the rest I would presume IS correct) :
-Waluigi = "Same age as Luigi" = same age as Mario, too
-Wario = NOT PROVEN to be related to Waluigi = may or may not be the same age
Super Mario Land 2 (I think?) :
-Wario = "childhood rivals" with Mario = around the same age. He "looks" older, though, but looks can sometimes be deceiving.
---
Speculation:
-Peach = "looks" "young" = at oldest, no less than 30 (or else she's doing a REALLY good job at not looking her age) = ^see under YI above = younger than Mario Bros. = hopefully not TOO much younger, or that would be a pretty big age difference between her and them, which wouldn't "seem right"
Daisy = nothing proven = "young" "looking" no less than 30 = around same age as Peach
----
These are just the ages in "Mushroom World Years" YI states that the incident which took place then happened a "long time ago," and it seems that in the past ~20 years, the characters have seemingly not aged.
--This means that most likely, time in the Mushroom World goes by much more slowly than in Earth (our) time. Their ages are based on what they would be in "Mushroom Time," and not EARTH actual time, since their time is obviously much, much different from ours, so we cannot speculate what their "Earth" ages would be.
I have a much deeper theory, but it is far too complex and time consuming to explain. You can already see how much time THIS has already taken, and it's just the "tip of the iceberg."
There you have it: a logical approach to something Shiggy didn't want to have to be bogged down with consistancy or logic. That's just the way my mind works. :P
--------------------
Wise men talk because they have something to say;
fools, because they have to say something.