Brahma’s age is of 100 years (divya varsh / god years).

Calculation of a divya varsh / god year

One day of Brahma = 1000 (one thousand) chaturyug (four yugas) and same is the duration of the night. 

{Note: - In one day of Brahma Ji, the term of rule of 14 Indras end. The term of rule of one Indra is 72 chaturyug. Therefore, in reality, one day of Brahma Ji is of 72 × 14 = 1008 chaturyug, and same is the duration of the night, but for sake of simplicity it is taken as one thousand chaturyug only.}

One Chaukri (Chaturyug) is made up of four Yugas which are as follows

1. Satyug, which is 1728000 years 
2. Treta Yug, which is of 1296000 years 
3. Dwapar Yug, which is 864000 years 
4. Kalyug, which is of 432000 years.

Month = 30 × 2000 = 60000 (sixty thousand) chaturyug

Year = 12 × 60000 = 720000 (seven lakh twenty thousand) chaturyug

Age of Brahma

720000 × 100 = 72000000 (seven crore twenty lakh) chaturyug

Age of Vishnu ji is seven times that of Brahma

72000000 × 7 = 504000000 (fifty crore forty lakh) chaturyug

Age of Shiv ji is seven times that of Vishnu

504000000 × 7 = 3528000000 (three arab 52 crore 80 lakh) chaturyug

Age of Brahm (Kaal / Jyoti Niranjan / kshar Purush) 

After the death of 70,000 (seventy thousand) Trilokiye Shiv, there is death of one Brahmlokiye Shiv i.e. Kaal (Brahm).

Age of Par Brahm (Akshar Purush)

When one Brahm (Kaal / Kshar Purush) dies it is one yug of ParBrahm. Such one thousand yugas make one day of ParBrahm (Akshar Purush) and same is the duration of a night. And then 100 years is the age of Par Brahm.

The bottom line is that despite having such huge ages, they are still in birth and death.