How old was Eren when he first transformed?

He was 10 when the colossal titan first attacked. 12 when he joined the training corps. 15 when he graduated from the training corps and when colossal titan re-attacked.

Why did all the Titans eat the smiling Titan?

The surrounding Titans, sensing Eren’s desire/unknowing command for her death, attack the Smiling Titan and devour her, allowing Eren and Mikasa to escape and avenging Carla and Hannes.

Why do Eren’s eyes remain the same colour throughout the series?

Finally we have Eren Yaeger who’s eyes remain green throughout. I believe his eyes remain the same colour because of his overwhelmingly powerful determination, ie. even after inheriting the founding titan his will remained his own.

What happens to Eren’s arms in Titan?

If a human emerges from their Titan form and receives severe injury, this person cannot transform into a Titan again until said injuries have healed We see Eren face off with the Armored Titan, but ends up being captured along with Ymir. They soon awaken with their arms severed off to prevent from the escaping.

Does the Founding Titan’s power affect your eye color?

If you are not part of the Reiss Family, the founding titan’s power impacts eye colour on a case by case basis. According the the Attack on Titan Wikia: When members of the Reiss family inherit the Founding Titan, they are affected by Karl Fritz’s will, and at times, their normally light-colored eyes darken and emit a glow.

What do the purple eyes of the Founding Titan mean?

Based on what is seen and known thus far, it would make sense that the purple eyes of the holder would signify that they are in control of the Founding Titan, when compared to other Titan Shifters, like Annie and Reiner. Evidence A — here — Look under the Titan Tab in the character details box.