The RMSS rules (RMFRP probably too) contain a Master Character Table T-5.8 (on pg.112). This table contains OBs, DBs and other skill bonuses for NPCs at lvl. 1, 5, 10, 15 and 20. Perhaps this helps.
I don't really understand the question about XPs because, well, at 5th lvl. an NPC, just like any other character, will have between 50000 and 59999 XPs. If you set the level then the XP don't really matter and should just follow the normal XP to level table.
I also don't really get what the questions about OB/DB or XP have to do with aging characters. Of course we might assume that an older character has experienced more in his life and might have a higher level than a youngster. A very old character might OTOH suffer from deteriorating stats. So you might reduce stats a bit as you please for old characters. Some old RM Companion book IIRC also contained some rules for old characters.