1
My dog is still very young.
Sign "dog," then use the sign for "young" by brushing your bent hands upward on your chest. You can emphasize the word "very" by making the upward brushing motion slightly faster, larger, or by widening your eyes.
2
When I was young, I loved to read.
Start by pointing to yourself for "I," sign "young," and then finish with "love" and "read." A slight nod or a nostalgic facial expression helps set the past tense context naturally for the person you are talking to.
3
She is the youngest in the family.
Point to the person for "she," then sign "young" followed immediately by the ASL suffix for "-est" (moving an 'A' handshape straight up high), and finish the sentence with the sign for "family."