Learn Finnish: Vanhin setä on lapsellisehko ja äreähkö. - The oldest uncle is somewhat childish and rather grumpy.

When adding the moderated degree suffix to adjectives ending in -nen, you attach it to the inflectional stem. For -nen words, you replace -nen with -se-. So lapsellinen (childish) becomes the stem lapsellise-, to which we add -hko.

Why does lapsellisehko end in -hko but äreähkö ends in -hkö?

This is due to standard vowel harmony. The suffix adapts to the vowels in the root word. Lapsellinen contains the back vowel a, so it takes the back-vowel suffix -hko. Äreä (grumpy) contains the front vowel ä, so it requires the front-vowel version -hkö.

Is there a difference in meaning between saying lapsellisehko and vähän lapsellinen?

The meaning is practically identical: both mean 'somewhat childish' or 'a bit childish'. Using the -hko / -hkö suffix is a stylistic choice that sounds slightly more concise and is very common in descriptive or written Finnish, though you will hear it in everyday speech too.

What form is vanhin in this sentence?