Think you should rename some of the keywords to more naturally suit their positioning.

Like, rather than

    [condition] if 
        <code> 
    [condition] else if 
        <code> 
    else 
        <code>

if you have

    [condition] then
        <code> 
    otherwise [condition] then
        <code> 
    otherwise 

(Though 'otherwise [condition] then' breaks your pure postfix style).

Similarly "while" could be "yields".

And "let" could be "over" (CB radio-style):

    10 = age over;