"Back in 1998"
*shows younger Abe*
SPACE IS WARPED AND TIME IS BENDABLE!
Here is how you need to approach the chronology of the show as we proceed forward into Season 26 and beyond.
Treat Springfield and the immediate surrounding area as a self-contained bubble where the "present" is concerned. Bart will always be 10, Lisa will always be 8, Homer & Marge will be whatever age they deem is pertinent for the episode at hand.
Everyone within the bubble remains the same age yet the world outside of the bubble continues to proceed as normal. Popular trends change. The Internet was conceived. Different presidents (Reagan, Bush Sr., Clinton, Bush Jr., Obama) all take office.
Mike Scioscia doesn't live within the bubble which is why he aged normally between Homer at the Bat in Season 3...
And MoneyBART in Season 22...
Their respective histories and futures will always course correct based on what the "present" year is currently perceived to be. There will be similarities and variations and a ton of chronological inconsistencies but for the people living within the "present" Springfield bubble will remain blissfully unaware of any changes that occur except for those that take place "within the bubble" such as Maude and Edna dying, etc. If someone dies "within the bubble" who wasn't meant to die - like a Fat Tony, Snowball II or Homer in Days of Future Future - events will course correct to designate a replacement.
As of right now, Lisa will believe she was born in the year 2006. Next year it will be 2007. Eight years from now it will be this year. Because she remains trapped within the temporal bubble. The only instance when time flows properly for the Simpsons and they actually age normally is when they travel outside of it - to Australia, Brazil (twice), Canada (umpteen times), Alaska (during the movie), etc.
Crazy events occur during these episodes because this bubble requires the Simpson family to live within it or it will break down and degrade. The Simpsons are either compelled to move back to Springfield or Springfield will move to where they are (as it did in At Long Last Leave).
And that is why Abe being younger in a flashback set in 1998 makes perfect sense.