Yordles are immortal 'aspect' type spirits while they have a 'true form' sort of, everyone perceives them as whatever that individuals 'belief'(assumption/prejudice/expectation) about what someone who embodies that aspect would look like.
Onto their age, using runtera's callender for 1 year (remember 'time' is an abstract concept measuring the numerical order of change, think '1 season' '1 year (1 repetition of the cycle of an orbit around the sun or 4 'seasons'), Teemo is likely in his 300s, Lulu Dissapeared '700 years ago' and some yordles remember her, prior to her lore update poppy was at least in her 300s as well.
For all creatures, maturity comes once you've acquired a certain number (different for each individual but the varience is fairly small) of certain types of key experiences, you can be 'mature' only once you've had the nessicary experiences, this is part of why Lulu is so immature despite eclipsing most yordles in being 'old' (tho I can't tell you how many 60-90s I've met with the maturity level of a tantrum throwing toddler, they never encountered any key growth experiences 'yet').
Trist and Teemo POPPY Heimer are all Veterans tho trust was once mentioned as being younger than teemo, she could be 25, 140, 240, 290 etc.
Such timeless creatures though it's fairly unimportant to assign arbitrary labels to them as they are mostly Homeostatic creatures, once they mature their 'aspect' they stay relatively stable (exceptions abound when isolated from others though). This kind of points to them being more bee/hive like and once separated they take on entirely different growth forms from standard yordles (just compare a domesticated animal, like a human, to a wild or feral counterpart).
All this stuff is just arbritary labeling and surprisingly isn't correlated with biological process or cyclical repetitions (indeed maturity is the anthesis of cycles and repetition as it's the BREAKING of cycles, habits and repitions in favor of new experiences and deep changes in somethings nature, and tho 'constantly looking at things with fresh/new perspective' SEEMS like a repetitious cycle it is not one (it really does seem like one though, but it's one of those 'can't be related to other things, because it is the only thing like itself' phenomena.).
,there's been something on my mind.
How old is Teemo?
being the most adorable.
