Age of Brahmaa

Pulastya Muni says - "Hey Bheeshm, Although Brahmaa is another form of Naaraayan, still his age is only 100 years according to the measurement of his own time. And Brahmaa's age is called "Par". Its half part is called "Paraardh" [Par + Ardh].

15 Nimesh = 1 Kaashthaa
30 Kaashthaa = 1 Kalaa
30 Kalaa = 1 Muhoort
30 Muhoort = 1 day-night
30 day-night = 1 Maas, or
2 Paksh = 1 Maas
6 Maas = 1 Ayan
2 Ayan = 1 year

There are two Ayan - Dakshinaayan and Uttarayan. Dakshinaayan is Devtaa's night, and Uttaraayan is their day. Thus --

1 Human year = 1 Divine day and night
360 Human years = 1 Divine year

12,000 Div years = 4 Yug (Sat, Tretaa, Dwaapar, Kali)

Sat Yug =  4,000 Div years + 800 Div years = 4,800 = 1,728,000 Human years
Tretaa =   3,000 Div years  + 600 Div years = 3,600 = 1,296,000 Human years
Dwaapar = 2,000 Div years + 400 Div years = 2,400 =   864,000 Human years
Kali =         1,000 Div years +  200 Div years = 1,200 =    432,000 Human years
      Total -------------------  =  4,320,000,000 human years  =  1 Chatur-Yug or Mahaa-Yug

In the beginning of each Yug, is the same number of years (in hundreds) is Sandhyaa, and in the end of each Yug is the same number of years (in hundreds) is Sandhyaansh. So in Sat Yug add 400 Div years of Sandhyaa, and 400 Div years of Sandhyaansh. Thus Sat Yug period is 4,800 Div years long. Thus any Yug is in fact is the middle period between Sandhyaa and Sandhyaansh. All these four Yug combined are called "Chatur-Yug" or "Mahaa-Yug". Such 1,000 Chatur-Yug make Brahmaa's one day. His night is also of the same number of years.

2,000 Chatur-Yug = Brahmaa's 1 day and night

This Brahmaa's one day has 14 Manvantar.
If Brahma's 1 day is of 1,000 Chatur-Yug, then there are 71 Chatur-Yug (out of 994 Chatur-Yug) plus 1/14th part of remaining 6 Chaturyug (a little less than 5,103 Div years) in 1 Manvantar.

1 Manvantar = 71 Chaturyug + 1/14th of 6 Chaturyug (a little less than 5,103 Div years)

1 Manvantar = 852,000 Divne years, or  306,720,000 Human years

Brahmaa's 1 day = 14 Manvantar or 4,294,080,000 + some more Human years = 4,320,000,000 Human years

[Brahmaa's one day is called one Kalp on Prithvi.]

Age of Saptarshi, Devtaa, Indra, Manu and His Sons

Since there are 14 Manvantar in Brahmaa's 1 day, there are 14 Manu (Lord of the Manvantar) in Brahmaa's 1 day. All - Saptarshi, Devtaa, Indra, Manu and Manu's sons, appear together in one Manvantar and all are destroyed together. Thus all these have the age of 71 Chaturyug + about 5,103 Divine years.

At the end of Brahmaa's day is Naimittik Braahm Pralaya (deluge). At that time Trilok (Bhoo Lok, Bhuvar Lok, and Swar Lok) start burning. Whoever live in Mahar Lok go to Jan Lok because of that heat. 

Brahmaa has the same number of years in his night also as his day. After the night passes, he starts his creation of the world all over again. Thus gradually, Brahmaa's age (100 years) passes by creating the world at the beginning of the day and destroying it at the end of the day."