I've been trying to find out their age when they died. I'm not really sure but I think they were in their twenties.
Both Minato and Kushina died at the age of 24.
Minato's birthday is estimated to be around the 1st of January/31st of December. Following this up using the timeline as provided on reddit
- Minato enters the academy at the age of 9, Kushina at the age of 8
- Minato became a Genin at 10, under Jiraiya's lead
- Kushina is abducted at the age of 13-14, and saved by Minato at around the same age
- The 3th Shinobi war starts, where Minato is around the age of 20.
- The Kannabi Bridge is destroyed by Minato Namikaze at the age of 23, shortly followed by the end of the war, where he also became the Yondaime
- Minato defeats Obito, seals Yin Kurama in his body and Yang Kurama in Naruto
- Minato dies at roughly at the age of 24, Kushina Uzumaki dies at age 24
These ages are based upon the mentioned dates mentioned through out the series, and the Naruto Wiki timeline.