For example, let's say I am lvl 5 and catch a Pidgey with a max cp of 150, and then lvl up to 10, will that Pidgey I had previously captured have a higher max cp now, or only the new ones I capture at lvl 10 have a higher max CP??
Your player level impacts the maximum CP level of all Pokemon, not just the ones you catch after levelling.
You can clearly see this mechanic by comparing the CP circle behind the Pokemon. After you level up, the "filled in" area will decrease, to reflect the increased maximum.
You may notice higher CP Pokemon also spawn, as your higher trainer level also allows you to catch higher CP Pokemon.
CP of all Pokemon scale with your character.
Every Pokemon has a (so far) unknown value that is considered their true "Max CP." At each Trainer Level, the game decides the Pokemon's current CP cap based on your level with a (so far) unknown formula.
However, the multipliers that are used to calculate how strong a Pokemon is are as follows:
L1: 0.094 L2: 0.16639787 L3: 0.21573247 L4: 0.25572005 L5: 0.29024988 L6: 0.3210876 L7: 0.34921268 L8: 0.37523559 L9: 0.39956728 L10: 0.4225 L11: 0.44310755 L12: 0.46279839 L13: 0.48168495 L14: 0.49985844 L15: 0.51739395 L16: 0.53435433 L17: 0.55079269 L18: 0.56675452 L19: 0.58227891 L20: 0.5974 L21: 0.61215729 L22: 0.62656713 L23: 0.64065295 L24: 0.65443563 L25: 0.667934 L26: 0.68116492 L27: 0.69414365 L28: 0.70688421 L29: 0.71939909 L30: 0.7317 L31: 0.73776948 L32: 0.74378943 L33: 0.74976104 L34: 0.75568551 L35: 0.76156384 L36: 0.76739717 L37: 0.7731865 L38: 0.77893275 L39: 0.784637 L40: 0.7903
The current theory (at the time of my editing this post) is that this is multiplied by a Pokemon's "Max CP" to get the value that decides their current CP cap. If this holds true, this table gives you an example of exactly how much CP levels scale up.
Pokemon in the wild will also be relatively stronger than they used to be. However, wild Pokemon will not increase in strength after you hit Level 30.
Data sourced from a data dump of mine