One of the Boys - 23
Teenage Dream - 25
PRISM - 3 days before turning 29

madonna 24
like a virgin 26
true blue 27
like a prayer 30
erotica 34
bedtime stories 36
ray of light 39
music 42
american life 44
confessions 47
hard candy 49
MDNA 53
rebel heart 56

she's untouchable

Alanis Morissette:
Alanis - 16
Now Is The Time - 18
Jagged Little Pill - 21
Supposed Former Infatuation Junkie - 24
MTV Unplugged - 25
Under Rug Swept - 27
Feast On Scraps - 28
So-Called Chaos - 29
Jagged Little Pill Acoustic - 31
The Collection - 31
Flavors of Entanglement - 33
Havoc and Bright Lights - 38

Brendon Urie (Panic! At The Disco)
A Fever You Can't Sweat Out - 18
Pretty. Odd. - 20
Live In Chicago - 21
Vices & Virtues - 23
Too Weird to Live, Too Rare to Die! - 26
Death of a Bachelor - 28

The ones in bold are actual studio albums released WW.

Rih:
Music Of The Sun - 17
A Girl Like Me - 18
Good Girl Gone Bad - 19
Rated R - 21
Loud - 22
Talk That Talk - 23
Unapologetic - 24
ANTI - 27 (released less than a month before her 28th birthday)

Britney
...Baby One More Time - 17
Oops!...I Did It Again - 18
Britney - 19
In The Zone - 21
Blackout - 25
Circus - Released on her 27th birthday
Femme Fatale - 29
Britney Jean - 31

Xtina:

Debut: 18
Stripped: 21
Back to Basics: 25
Bionic: 29
Lotus: 31

Gwen and No Doubt:

Debut: 22
Beacon Street Collection: 25
Tragic Kingdom: 26
Return of Saturn: 30
Rock Steady: 32
L.A.M.B: 35
The Sweet Escape:37
Push and Shove: 42
This is What the Truth Feels Like: 46

Hannah Montana - 13
Hannah Montana 2: Meet Miley Cyrus - 14
Breakout - 15
The Time of Our Lives EP, HM 3, HM The Movie - 16
Can't Be Tamed, HM Forever - 17
Bangerz - 20
Miley Cyrus & Her Dead Petz - 22

Queen Vanessa Carlton

Be Not Nobody - 21
Harmonium - 24
Heroes & Thieves - 27
Rabbits on the Run - 30
Liberman - 35

...Baby One More Time - 17
Oops!...I Did It Again - 18
Britney - 19
In The Zone - 21
Blackout - 25
Circus - 27
Femme Fatale - 29
Britney Jean - 31

Shakira (1977)

Magia - 14
Peligro - 16
Pies Descalzos - 18
Dónde Están Los Ladrones? - 21
Laundry Service - 24
Fijación Oral Vol. 1 - 28
Oral Fixation Vol. 2 - 28
She Wolf - 32
Sale El Sol - 33
Shakira. - 37

Tiziano Ferro:

Rosso Relativo - 21
111 Centoundici - 23
Nessuno è solo - 26
Alla mia età - 28
L'amore è una cosa semplice - 31
TZN - The Best of Tiziano Ferro - 34
TZN - The Best of Tiziano Ferro (Lo stadio Tour 2015 Edition) - 35
TBA - 36

Looks like if they start releasing serious stuff by their 30s they flop. Interesting.