[Bluebull]

Yes it cannot be 100 years
Let me tell you a story...
MSW does not originate from Pahang, but actually from Kelantan 
And was named after a small town there, called Gua Musang
In 1985 or there about, a farmer owner of Fook Gor Farm from Pahang went to Kelantan 
He came across a very old durian tree near Gua Musang
He brought back a branch from this old tree to his farm in Raub, Pahang and started propagating it
Seven years later, the tree bear fruits
He and his fellow farmers tasted it and found it to be like tasted like heaven 
At that time, the naming and branding of durians was getting popular
So he named it and registered it as Raja Musang or Musang King
Later it evolved to become Mao Shang Wang 
Even the original Chinese name is different from the present one (Mao becomes cat)

In the early 2000s it was first introduced to Singapore
But it was not as popular as the then Sultan Durians and price was Low
Then came the cock and bull stories in the Hong Kong media about Stanley Ho and Li Ka Shing in 2010
They brought in their private jet to fly in MSW durians they bought from Singapore
The next day prices rocketed and remain high till today

When you are talking about Old Tree MSW from Pahang
You are probably talking about the descendants of these trees from Fook Gor plantations
So the durians are not necessarily from the original old tree but are from the seeds of the old tree
So could be only 5 to 10 years old
Note that last time durian trees took 7 years to bear fruits
Nowadays through speedy propagation and genetic fertilisation 
Durian trees can bear fruits within 4 to 5 years