I'm 39.
My oldest pokemon is a 3080 Gary Level 36 89% with DB/DP that was my first Gary. It's never got much use other than sitting high up in gyms in the old system
My second oldest is a 2350 Blissey that was my first chansey hatched. When gen 2 came out I evolved, powered up, realized that despite its amazing stats its low Attack was horrible for a blissey and promptly worked on better options.
My 3rd Oldest is 1473 Magneton 87% 15 Attack. It was a spark discharge and got a little work as a prestiger in that brief period after discharge got buffed and before the gyms got nerfed
My 4th oldest is a 82% Level 38 Vaperon that has won thousands of battles. There is no way I would power this pokemon up now a days, but I was 36 or 37 when Gyms changed and this was powered up enough to sit above shaving line in my area.
My 5th oldest is a 93% Flareon that got Heat Wave. It never got transferred or powered or TMed or ever around so it sits forgotten.
My 6th oldest is a Level 32 96% Dragonite . Originally rolled with SW/DP and sat in many a gym. I Fast TMed to DT, but have held off on the charge TM until Gen3 comes out. In case Gen 3 comes out with another rebalance.
My 7th oldest is 98% Nidoqueen with 2 poison moves that is level 21. Powered it up once before I realized it sucked.
My 8th oldest is a 91% 15 attack level 18 Dratini. I've never gotten around to investing in this dratini. Ah well perhaps someone would trade for it one day
My 9th oldest is my first machamp, which ironically was one of the last 3 things in my Gen1 dex. It's only 84% so I never powered it up but it has legacy KC/Submission set.
10th place goes to Bite Hydro Gary. He's level 38.5 with 98% IV (sniff 14 attack). But this pokemon since I got him last August was a staple in many of thousands of gyms I defeated back in the day.
Feel free to add your own stories of your....