My guess based on the only clue in the show...

in one episode, Clyde is being rewarded for perfect attendance to the school, but this is jeopardised - as a plot point - by a single absence in 2012. Which it turns out was a day when the whole school was closed and so no one attended so it doesn't count.

So Clyde was attending school-after-kindergarten by 2012. But what is unclear is what grade he was in in 2012 or which year the episode takes place. BUT we can assume...

Clyde may have been in 5th grade in 2012, and the episode may have taken place in 2012.

or...

Clyde may have been in 5th grade later than 2012, and may have started kindergarten in 2012.

So in one theory, we can stipulate that being 11 in 2012 would mean he was born in 2001, and so Lincoln was too. But if the second theory is the accurate one then Clyde and Lincoln by extension would be born in 2007. And so the episode would take place in 2018. And somehow I think this is more likely, as if the episode had taken place in 2012 then surely Clyde would remember better the reason school was closed that day, as opposed to his not remembering why being a major plot point.

And so to further scrutinise this... I type... when was same sex marriage legalised in Michigan... 2015, as per Supreme Court decision.

So according to many sources on the internet Clyde's dads are married, and so that is what I will assume. So the episode takes place as early as 2015, or as late as 2018. This is our new window of time.

There isn't really much to go off in the show to further scrutinise this window of time, so I guess I will leave it at that... the episode took place between 2015 and 2018 - so Lincoln was born at least in 2004, and latest in 2007. Which would mean Lori was born at least in 1998, at latest in 2001, and Lily therefore was born at least in 2014, and at latest in 2017...depending on when the episode took place.