I do not understand why in Spanish the following sentence makes sense

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> Cuando Jaime cumpla 50 años, vamos a darle una fiesta. 
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> 
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but this does not?

> 
> Cuando Jaime cumpliere 50 años, vamos a darle una fiesta
> 
> 
> 

Since we are referring to Jamie, don't we use the future tense of el/ella instead of the present tense of yo?