Which of the following two options is the correct beginning of sentence in Spanish?:
- 30 años después...
- 30 Años después...
First, note that in Spanish, as in English, is recommended not to start a sentence with a number (This is because digits are just a quantity but the word is the actual name of the number). The easiest way of following this rule would be writing the number with letters.
Treinta años después...
or try to write the sentence in a different way
Unos 30 años después...
Después de 30 años...
Thus said, you would not need to capitalize the word following the number since is not the beginning of the sentence (of course there are other rules that you'll have to abide: capitalize it if it is a proper name, etc.). So the answer, if you were forced to start your sentence with a number (for example in a short line similar to your example in a slide for a presentation or the like), would be:
30 años después...
For example, you have some slides for a presentation in which you are describing some achievements or fact of your company:
20 años de experiencia en el sector
500 puestos de trabajo creados
The way that I would do it is
Treinta años despúes
because you do not write numbers in Spanish sentences so you write it in letters.
Since the number is written in letters,it has to be capitalized. Now you don't need to capitalize años.
Here are some examples:
Mi hermano tene diecinueve
Yo quiero dos libros