The Date Place is available from the start in both the 4U and the 4U+, however, it's pretty useless until your Tama reaches a certain age. Like, for example, even immediately after a teen ages into an adult, your Tama will only be able to visit the Date Place, but you will not have the option to marry off your Tama. Instead, I've only been able to marry off my Tama at the Date Place once it unlocks the age Tamatomo which all Tamas achieve after being an adult for 48 hours. In the 4u+, there is the Encounter Street, which is unlocked after acquiring a few Tamatomos over some generations. Encounter Street works similarly dating Tamas at Bud Park in the P's, where you can encounter a character and go on dates with them, and after going on enough dates with them, you'll be prompted to marry them. This is the only way that I know of that will marry off your Tama sooner than waiting for it to achieve it's 48-hours-as-an-adult Tamatomo.
So, if this theory is correct: your Tama is a baby for 1 hour, and then an adolescent for 48 hours, and then achieves it's age-centered Tamatomo after being an adult for 48 hours; you should be able to marry off your Tama after 97 hours after its birth. This is how I understand marriage and moving towards new generations in the 4U and the 4U+, but I haven't experimented beyond this theory, so it may or may not be sound.
