can you guess what it is?

when Juan was 2, Pedro was 1, therefore he is 1 year older than Pedro, so that when Juan is 100 Pedro is 99

Juan was 2 and Pedro was 1.

so, if Juan is 100 Pedro is 99

When Juan was $2$ years old Pedro is $2 \times$ $\dfrac{1}{2}$ $= 1$ years old

So Pedro is $1$ year younger than Juan

$∴$ when Juan is $100$ years old Pedro is $100 - 1 = 99$ years old

When Juan was 2 years old Pedro was $\frac{1}{2}$ Of Juan's age or 1 year old

so:

2-1=1(difference between Juan's and Pedro's age)

so when Juan is 100

100 - 1 = 99

The difference is always $1$ .

Answer: $99$