Short answers, pulled from the story.
As of the early 21st century, approximately 800 million people spoke Indo-Aryan languages, with estimates for total speakers ranging as high as 1.5 billion. The largest single variety is Hindustani, spoken by around 330 million people as a first language.
The oldest directly attested Indo-Aryan is Vedic Sanskrit, used in the foundational Hindu sacred texts known as the Vedas. Comparable in age is the Indo-Aryan superstrate found in Mitanni records from around 1400 BCE, where royal names and horse-training terminology closely match Sanskrit words.
Romani is classified as a New Indo-Aryan language, meaning proto-Romani speakers could not have left India significantly earlier than around AD 1000. Scholarly analysis by Pott in 1845 and Miklosich between 1882 and 1888 established this, and shared patterns of gender reduction between Romani and other New Indo-Aryan languages support the conclusion that its speakers remained on the subcontinent until a relatively late period.
The Shravakacara of Devasena, dated to the 930s, is now considered to be the first book written in Hindi. It emerged during the Apabhramsha period, when transitional dialects were bridging Middle and early modern Indo-Aryan.
Modern Standard Hindi and Modern Standard Urdu were both derived from Dehlavi-based Hindustani, but received separate official status following the division of the British Indian Empire in 1947. Hindi became the official language in India; Urdu became official in Pakistan.
First proposed by Rudolf Hoernle in 1880 and later refined by George Grierson, the Inner-Outer hypothesis argues that Indo-Aryan has a core and a periphery. Outer Indo-Aryan represents an older stratum mixed to varying degrees with a newer Inner stratum. The most recent version, by Franklin Southworth and Claus Peter Zoller, is based in part on an Outer past tense marked with the suffix -l-.