Mamma: "Let me think, Tommy. Well, our three ages add up to exactly seventy years."

Tommy: "That's a lot, isn't it? And how old are you, Papa?"

Papa: "Just six times as old as you, my son."

Tommy: "Shall I ever be half as old as you, Papa?"

Papa: "Yes, Tommy; and when that happens our three ages will add up to exactly twice as much as to-day."

Tommy: "And supposing I was born before you, Papa; and supposing Mamma had forgot all about it, and hadn't been at home when I came; and supposing..."

Mamma: "Supposing, Tommy, we talk about bed. Come along, darling. You'll have a headache."

Now, if Tommy had been some years older he might have calculated the exact ages of his parents from the information they had given him. Can you find out the exact age of Mamma?

Mamma's Age. Amusements In Mathematics by Henry Ernest Dudeney (1917).

Answer:
The age of Mamma must have been 29 years 2 months.

That of Papa, 35 years; and that of the child, Tommy, 5 years 10 months. Added together, these make seventy years. The father is six times the age of the son, and, after 23 years 4 months have elapsed, their united ages will amount to 140 years, and Tommy will be just half the age of his father.

The answer above is taken from the original book, here is another version of the answer:

If we call Tommy T, Mamma M and Papa P we can see that:

"our three ages add up to exactly seventy years" leads to

T + M + P = 70 (1)

"Just six times as old as you" leads to

P = 6 x T (2)

In an unknown number of years (Y) "Shall I ever be half as old as you" leads to:

P + Y = 2 x (T + Y) (3)

and "our three ages will add up to exactly twice as much as to-day" leads to:

(T + Y) + (M + Y) + (P + Y) = 140

which can be written as

T + M + P + 3Y = 140 (4)

We can see from (4) and (1) that

3Y = 70

so

Y = 70 ÷ 3 (5)

Using (2) and (5) in (3) we have

P + Y = 2 x (T + Y)

6 x T + 70÷3 = 2 x (T + 70÷3)

4 x T = 70÷3

T = 70÷12 (6)

We can now use (6) in (2) to see that:

P = 6 x T

P = 6 x 70÷12

P = 70÷2

And using the values for T and P in (1) we have:

T + M + P = 70
70÷12 + M + 70÷2 = 70

Multiply throughout by 12 to give:

70 + 12 x M + 420 = 840

12 x M = 840 - 420 - 70

12 x M = 350

M = 350÷12

So: Tommy = 70÷12 = 5.83333 = 5 years 10 months.
Papa = 70÷2 = 35 = 35 years.
Mamma = 350÷12 = 29.1666 = 29 years 2 months.