I was looking at some of my favourite manga series and other popular titles to see how old the authors were when they started to write them, and it was a bit of a wake up call for me, especially the younger ones.
Rounding the years roughly, this is how old the authors were when making these series:
FMA - 27
Bleach - 24
Naruto - 25
Rave Master - 23
^same author, Fairy Tail - 29
One Piece - 22
Rurouni Kenshin - 24
History's Strongest Disciple Kenichi - 25 (probably)
Nichijou - 29
JoJo's Bizarre Adventure - 27
Lucky Star - 26
Kimi ni Todoke - 30
I know that for around 6 years I didn't have writing a manga as a main goal or even having art as a career option, and I deliberately kept it as a background hobby for longer than I "should have", given my current desires.
I'm not saying that if you're older you can't write manga (there's plenty of stuff written by older people that brilliant), but this is, amongst other things, the level of a pro mangaka. Yes they trained and went to art school and apprenticed under an already pro mangaka, so they've got a lot going for them.
Basically, to me, this relates to my desired life output and what I want to do in terms of quantity, for my entire life.
I definitely want to be a good artist by the time I hit 30 (2 years to go :|), only just because it's a round number and viewed as a checkpoint of sorts by a lot of people, and timing-wise I might just scrape in :P
I'm not saying their ages should be a target for aspiring manga writers, but merely that this is what is achievable if you put your mind to it, are dedicated and focused.
Here are some "older" artists that have equally started writing popular works (possibly as their second/third series)
Nanatsu no Taizai - 35
Ore Monogatari!! - 40
World Trigger - 32
Detective Conan - 31
Black Clover - 31
As a side point most of these artists have written one-shots that you've never heard of and serials that got cancelled through lack of popularity etc, so it's not like they had it easy, they had to work hard to get where they are and deal with disappointment and setbacks as well.
Basically, keep at your goal, if you're young, make good use of your time, if you're older, make good use of your time. That's all.
Onwards and upwards.