वयसि प्रथमे

The affix ङीप् is used in the feminine after a प्रातिपदिक (Nominal Stem) ending in अ and denoting early age.
The word वयः means the condition of the body as dependent upon time; such as, youth, old age &c. Words expressive of early age form their feminine by ई, though ending in अ ॥ Thus, कुमारी 'a girl'. किशोरी 'a young girl' बर्करी 'a she-kid'.

Why do we say denoting 'early age?' Observe स्थविरा 'an old woman'; वृद्धा 'an old woman'. Why do we say ending in अ ? Observe शिशु, which is both masculine and feminine.

Vart:- The sutra should have been आचरमे वयसि 'words expressive of not old age'? Thus वधूटी 'a young woman', चिरण्टी 'a young woman'. These words denote the second stage of life, and not the first, i. e. they denote women who have attained their maturity.

How do you explain the form कन्या 'a girl'? This is an exception, and Panini himself uses the word 'kanya' showing that it is a valid form; as in sutra 4.1.116.

The words like उत्तानशया 'an infant girl that does not know how to lie down properly' लोहितपादिका 'a female infant whose feet are still tender and red' do not directly, but metaphorically, express age, and hence are not governed by this rule.